Allow a U2F Device to be the Second Factor for Authentication

Parent Issue: #15337 

## TODO
- [ ] #15337 (!3905) FIDO/U2F 2FA using Yubikey
    - [x] Order a Yubikey?
    - [x] Do some reading to figure out what all this stuff means
    - [x] Look through the existing MR
    - [x] Browser support?
    - [x] Implementation
        - [x] User can register 2FA using their U2H device instead of authenticator
            - [x] Barebones flow
            - [x] Save the registration in the database
            - [x] Authentication flow
            - [x] First try after login/server start doesn't work
        - [x] User can log in using their U2F device
        - [x] Allow setting up authenticator if U2F is already set up (or vice versa)
        - [x] Change `two_factor_auths/new` to `show`
        - [x] `sign_requests` during registration? (Registering a device that has already been registered)
        - [x] 2FA skippable flow?
        - [x] Enforced 2FA flow (grace period?)
        - [x] Move the "Configure it Later" button to the right place
        - [x] Don't allow registration when the yubikey isn't plugged in
        - [x] Polish authentication flow
        - [x] Login should only show the 2FA method that's enabled
            - [x] Message to say that u2f only works on chrome, and it's recommended to enable otp as well.
        - [x] Index for key_handle
        - [x] Server-side errors while registering/logging in
        - [x] Handle non-chrome browsers
        - [x] Try to authenticate with a key that hasn't been registered (shouldn't work)
        - [x] Try the same key for multiple user accounts (should work)
        - [x] Fix existing tests
        - [x] Make sure CI is green
        - [x] Add tests
            - [x] Figure out how to fake the Yubikey
            - [x] Teaspoon tests for the React components
                - [x] Each device can only be registered once per user
            - [x] Feature specs
                - [x] Regular flows
                - [x] Test error cases
        - [x] Refactoring
            - [x] Refactor App ID
            - [x] Clean up the `show` action
        - [x] Annotate methods with definition of U2F
        - [x] Changelog
        - [x] Fix merge conflicts
        - [x] Verify flows
            - [x] Authenticator + no U2F
            - [x] U2F + no authenticator
            - [x] U2F + authenticator
            - [x] U2F + authenticator -> disable 2FA
            - [x] 2FA required with different grace periods
        - [x] Screenshots for MR
    - [x] Augment the [help docs](http://localhost:3000/help/profile/two_factor_authentication)
    - [x] Assign to endboss
    - [x] Ask for feedback on UI/UX
    - [x] Ask for feedback on copy
    - [x] Wait for review/merge
    - [x] Fix merge conflicts
    - [x] Wait for CI to pass
    - [x] Implement review comments/suggestions
        - [x] Move `TwoFactorAuthController#create_u2f` to a service
        - [x] Extra space before `Base64` in `u2f_registration` model
        - [x] Move `with/without_two_factor` scopes to class methods
        - [x] In `profiles/accounts/show`, add spaces at `{` and `}`
        - [x] Remove blank lines in `profiles/two_factor_auths/show`
        - [x] Fix typo in doc. "(universal 2nd factor )"
        - [x] Add "Added in 8.8" to doc
        - [x] In the doc, use 'Enable 2FA via mobile application' instead of 'Via Mobile Application'
        - [x] In the doc, use 'Enable 2FA via U2F device' instead of 'Via U2F Device
        - [x] Use "Two-Factor Authentication" everywhere
        - [x] Use `#icon` wrapper instead of `fa_stacked_icon`
        - [x] Check if `string` is enough for `key_handle` and `public_key`
        - [x] Separate `exercise` and `verify` phases of test (u2f_spec)
        - [x] Assert that `user_without_2fa` is _not_ in results (with_two_factor)
            - [x] Remove rubocop exception
        - [x] Refactor call to `User.with_two_factor.count` to not include `.length`
        - [x] Add a note that makes the "Disable" button/feature obvious
        - [x] Remove i18n
        - [x] Test in Firefox with addon (+ create new issue for support)
        - [x] Remove React
            - [x] Rewrite registration
            - [x] Switch underscore template to default style
            - [x] Rewrite authentication
            - [x] Move `register` haml to `u2f` dir
            - [x] Remove instance variables
            - [x] Fix tests
            - [x] Read SCSS guidelines
            - [x] Address @connorshea's comments regarding text style
            - [x] Make sure all classes and IDs are in line (add `js-` prefixes)
                - [x] Register
                - [x] Authenticate
            - [x] Refactoring?
        - [x] Include non-minifed version of bowser
        - [x] Audit log
        - [x] Look at the `browser` gem (and don't use bowser)
        - [x] Error message when on HTTP?
    - [x] Test on Mobile
    - [x] Fix merge conflicts
    - [x] Retest all flows
    - [x] Back to Rémy for review
    - [x] Make sure CI is green
    - [x] Wait for merge / more feedback
    - [x] Implement @rymai's changes
        - [x] JS/Coffeescript variables should be lowerCamelCase
        - [x] Spaces before/after `}` and `{` in HAML (and elsewhere)
        - [x] Rails view helpers in u2f HAML
        - [x] `%div.row.append-bottom-10`
        - [x] Wrap line in `without_two_factor` scope
        - [x] Exception-less flow in `U2F::CreateService`
    - [x] Fix merge conflicts
    - [x] Move service to model class method
    - [x] Fix teaspoon specs
    - [x] Address @rymai's suggestions about error handing
    - [x] Javascript error constants
    - [x] Fix merge conflicts
    - [x] One final review
        - [x] Test "registration with errors" flow
    - [x] Assign to Remy
    - [x] Wait for replies from @jschatz1
    - [x] Address @rymai's comments
        - [x] Omit `%div`
        - [x] Scope `$.find` globally
        - [x] Replace `find('#element-id).click` with `click_on('Element Text')
    - [x] Rebase master + conflicts
    - [x] Look at https://news.ycombinator.com/item?id=11690774
    - [x] Address @connorshea's comment regarding HTTPS on localhost
    - [x] Final sanity check
    - [x] Wait for [CI to pass](https://gitlab.com/gitlab-org/gitlab-ce/commit/c84179ad233529c33ee6ba8491cfea862c6cd864/builds)
    - [x] Address @rymai's next round of comments
        - [x] Interpolate `true` and `false` in DB scopes
        - [x] Why have `Gon::Base.render_data` thrice?
        - [x] `user_spec` should have correct spacing
        - [x] Use `arel_table[:id]` instead of `users.id`
        - [x] URL helper in `app/views/profiles/two_factor_auths/show.html.haml`
        - [x] Remove polyfill change
    - [x] Wait for [CI to pass](https://gitlab.com/gitlab-org/gitlab-ce/commit/0123ab8/builds)
    - [x] Address @jschatz1's comments
        - [x] Use `on('click', ...)` instead of `click(...)`
        - [x] Use `is` and `isnt` in coffeescript
        - [x] Use `and` and `or` in coffeescript
    - [x] Add `Gon::Base.render_data` to `devise_empty` (and other base layouts)
    - [x] Wait for [CI to pass](https://gitlab.com/gitlab-org/gitlab-ce/commit/401916397336174c582be3d3004a072f845d4b5f/builds)
    - [x] Wait for [build](https://gitlab.com/gitlab-org/gitlab-ce/commit/75955710ef9a5f0dcee04e8617028c0e3ea5bf50/builds) to pass
    - [x] Fix merge conflicts
    - [x] Inspect diff / workflow
    - [x] Assign back to @rymai
    - [x] Make sure [ci](https://gitlab.com/gitlab-org/gitlab-ce/commit/2c6316b29a9276ef44c7b4b39363a611bf5973a6/builds) has passed
    - [x] Fix merge conflicts (probably introduced by [devise upgrade](https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4216)
    - [x] Wait for [CI](https://gitlab.com/gitlab-org/gitlab-ce/commit/a5ef48b7aa63d0d9e45b41643043b57208eaab9f/builds) to pass
    - [x] Respond to @rymai's comments
        - [x] Use `elsif`
        - [x] Check if we need `and return`
        - [x] Only fetch key handles from the DB
        - [x] No annotations to models?
        - [x] Align hash keys in model
    - [x] Wait for [build](https://gitlab.com/gitlab-org/gitlab-ce/commit/e0ef504734e7f14813c73bbb79f5c5f6fae3248c/builds) to pass
    - [ ] Wait for merge

## Screenshots

![Screenshot_2016-05-03_09.53.04](/uploads/1af3f277efa488dc107d36e6b4b07ca4/Screenshot_2016-05-03_09.53.04.png)
![Screenshot_2016-05-03_10.19.53](/uploads/2bfc67dfb96c0e005cce033d8b456813/Screenshot_2016-05-03_10.19.53.png)
![Screenshot_2016-05-03_10.19.56](/uploads/e912abedd5b1d07d7185cee9f204c5ff/Screenshot_2016-05-03_10.19.56.png)
![Screenshot_2016-05-03_10.20.04](/uploads/9350d5c98823d1f3d4e59517dfb8910a/Screenshot_2016-05-03_10.20.04.png)
![Screenshot_2016-05-03_10.31.15](/uploads/84473dc263e0643311a39006e649035f/Screenshot_2016-05-03_10.31.15.png)
![Screenshot_2016-05-03_10.31.22](/uploads/13ce43e0d7a565000af29984667eeb08/Screenshot_2016-05-03_10.31.22.png)
![Screenshot_2016-05-03_10.31.37](/uploads/b90fbb40dbf9bbd73af324f48ffdc948/Screenshot_2016-05-03_10.31.37.png)
![Screenshot_2016-05-03_10.36.48](/uploads/41a0fbc493c6fefeafd922b3ddf2a25e/Screenshot_2016-05-03_10.36.48.png)

See merge request !3905

- "two-factor" for OTP-based 2FA
- "two-factor-via-u2f-device" for U2F-based 2FA
- "standard" for non-2FA login

- Move the `authenticate_with_two_factor` method from
  `ApplicationController` to the `AuthenticatesWithTwoFactor` module,
  where it should be.

- Move the `TwoFactorAuthsController`'s `new` action to `show`, since
  the page is not used to create a single "two factor auth" anymore. We
  can have a single 2FA authenticator app, along with any number of U2F
  devices, in any combination, so the page will be accessed after the
  first "two factor auth" is created.
- Add the `u2f` javascript library, which provides an API to the
  browser's U2F implementation.
- Add tests for the JS components

- Turbolinks caches the `head`, so `gon` updates don't show up unless
  the user navigates to page directly (by URL) or performs a refresh.
- The solution is to render `gon` in the body instead.
- Also update the syntax to the new Rails 4 (according to the gon
  README) syntax.

- Need the `mobile?` detection (that the new version provides) for the
  U2F registration/ authentication flow

- To hold registrations from U2F devices, and to authenticate them.
- Previously, `User#two_factor_enabled` was aliased to the
  `otp_required_for_login` column on `users`.
- This commit changes things a bit:
    - `User#two_factor_enabled` is not a method anymore
    - `User#two_factor_enabled?` checks both the
      `otp_required_for_login` column, as well as `U2fRegistration`s
    - Change all instances of `User#two_factor_enabled` to
      `User#two_factor_enabled?`
- Add the `u2f` gem, and implement registration/authentication at the
  model level.

Fix rubocop offense in awardable specs

Fixes failing tests on master.

See merge request !4481

Enable Lint/AmbiguousOperator rubocop cop

Enforce using parentheses if ambiguous operators are detected, like `header *some_method` or `array.select &:even?`.

See #17478

See merge request !4454

[ci skip]

Document the API endpoint for gathering a build log

## What does this MR do?

Adds documentation for gathering a trace build log via the API. Documenting the following:
https://gitlab.com/gitlab-org/gitlab-ce/blob/de20bd5b31715f096db3fb0155c82b0eea992b6c/lib/api/builds.rb#L98-110

## Are there points in the code the reviewer needs to double check?

Please verify that a curl request using the same format as documented works

## Why was this MR needed?

No documentation currently exists

## What are the relevant issue numbers?

#18015 

## Screenshots (if relevant)

See merge request !4467

Fix link to current design reference in the CONTRIBUTING guide

## What does this MR do?

It fixes an outdated link by replacing it with the up-to-date one.

See merge request !4439

Allow bulk-assign labels to issues

- [x] Allow bulk-assignment labels.
- [x] Show indeterminate state for labels present on selection.
- [x] Remove existing labels from selected items if label gets unmarked.
- [x] Fix conflicting tests.
- [x] Write tests.

Closes #13499 and #15489

See merge request !3917

Instead of passing `remove_label_ids`, just pass an empty array for
`label_ids` (and don't pass `add_label_ids` or `remove_label_ids`).