API: Expose subscribed? on issues and merge requests

This is the first start for the new `subscribable` API. It exposes the `subscribed` field for issues and merge requests.

Related to #6024

See merge request !3611

Fix Incorrect Quote In Docker Executor Example

## What does this MR do?
The corrected line opened with a regular quote (') but closed with a back-tick (`)  
I believe this is invalid bash syntax

## Are there points in the code the reviewer needs to double check?
Probably not

## Why was this MR needed?
Fixes error when example is copied and pasted

## What are the relevant issue numbers?
None

## Screenshots (if relevant)
None

See merge request !3604

Remove changelog entry for new navigation sidebar.



See merge request !3608

Fix side-by-side code format & commit message wrap

![Screen_Shot_2016-04-07_at_1.31.28_PM](/uploads/bad00284e4dfbec1fdd75220c34f4a98/Screen_Shot_2016-04-07_at_1.31.28_PM.png)

![Screen_Shot_2016-04-07_at_1.32.23_PM](/uploads/7cd344765025e93d0035934a473b4bb3/Screen_Shot_2016-04-07_at_1.32.23_PM.png)

See merge request !3605

Revert "Merge branch 'new-navigation-prototype' into 'master'"

This reverts merge request !3494

See merge request !3607

[ci skip]

This reverts merge request !3494

Revert "Merge branch 'fix-sidebar-exapnd' into 'master'"

This reverts merge request !3520

See merge request !3606

This reverts merge request !3520

Preserve white space



See merge request !3602

Update number of Todos in the sidebar when it's marked as "Done"

Closes #15002 

See merge request !3600

Fix problem when creating milestones in groups without projects

Fixes #14012 

See merge request !3481

Add optional colon.



See merge request !3591

Disable git gc --auto

See merge request !3572

Hide "assign to me" link if not allowed

Fixes #14996

See merge request !3590

* 'master' of dev.gitlab.org:gitlab/gitlabhq:
  Make sessions controller specs more explicit
  Fix 2FA authentication spoofing vulnerability
  Add specs for sessions controller  including 2FA

Fix 2FA authentication spoofing

## Summary

This is security fix for vulnerability described at 
https://gitlab.com/gitlab-org/gitlab-ce/issues/14900.

Attacker was able to bypass password authentication of users that have 2FA enabled, and consequently sign is as a different user, without knowing his password, if he managed to guess 2FA One Time Password for that user.

It was also possible to enumerate users and check if they have 2FA enabled, because GitLab responded with different error for each case.

## Fix

This MR attempts to change default user search scope if `otp_user_id` session variable has been set. If it is present, it means that user has 2FA enabled, and has already been verified with login and password. In this case we should look for user with `otp_user_id` first, before picking it up by `login`.

Both, 2FA authentication spoofing and 2FA discovery have been covered by specs.

## Further work

Current 2FA code is a bit tricky, so it probably needs some refactoring.



See merge request !1947

Expire caches after project creation to ensure a consistent state

See merge request !3586

Only update main language if it is not already set

Related to gitlab-org/gitlab-ce#14937 (but does not fully fix) This is a temporary fix so performance isn't affected so much. 

cc @yorickpeterse @ayufan how does this look?

See merge request !3556

This commit attempts to change default user search scope if otp_user_id
session variable has been set. If it is present, it means that user has
2FA enabled, and has already been verified with login and password. In
this case we should look for user with otp_user_id first, before picking
it up by login.

API: Ability to filter milestones by state

Ability to filter milestones by `active` and `closed` state.

* Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/14931

See merge request !3566

Expose badges

This MR exposes badge somewhere in visible place.

![expose_badges](/uploads/d2e290d3013d1ef2b1bdeebbbe2c5d8b/expose_badges.png)

Closes #13801

See merge request !3326

Fixes #14638.

The SQL query was ambiguous and in this case we want to filter projects.

See merge request !3462

Return status code 303 after a branch DELETE operation to avoid project deletion

Closes #14994

See merge request !3583