Move issuable title form to a new partial

Continue to refactor issuable form view (after !7249).

Part of #23864.

See merge request !7453

Signed-off-by: Rémy Coutable <remy@rymai.me>

User avatar in mention autocomplete in comment box

## What does this MR do?
This MR shows avatar in `@<username>` autocomplete in comment box. Also shows first latter as avatar if there is no group avatar is available.

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

## Why was this MR needed?
This MR closes issue  #22790 
 
## Screenshots (if relevant)
**Before**
![Screen_Shot_2016-10-25_at_17.44.44](/uploads/c44de668c7a2d482594423ebcc917440/Screen_Shot_2016-10-25_at_17.44.44.png)
**After**
![mention-avatar](/uploads/7cd2fbefa42e4a5f14335e057cf6978a/mention-avatar.png)
## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [x] API support added
- Tests
  - [x] All builds are passing
- [ ] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?


Closes #22790

See merge request !6865

Copy messaging guidelines

Update documentation to start having consistency and guidelines for copy (messaging) in GitLab.

See merge request !7467

Add new file for copy (messaging) guidelines

Update copy guidelines

Fix typo

Add forms to guidelines

Simplify Copy heading

Refine copy page. Add images and table

Fix toc link on Copy page

Clarify LDAP troubleshooting ldap_search example

A customer noted an error/lack of clarity in the LDAP documentation
    with the `ldap_search` example. Previously, if taken literally, the
    customer may have expected the `$` variables to be automatically
    replaced or if they paste the exact `user_filter` contents the
    parentheses would have been incorrect. Let's just simply the filter
    and use exactly what's in the configuration.

See merge request !7489

A customer noted an error/lack of clarity in the LDAP documentation
with the `ldap_search` example. Previously, if taken literally, the
customer may have expected the `$` variables to be automatically
replaced or if they paste the exact `user_filter` contents the
parentheses would have been incorrect. Let's just simply the filter
and use exactly what's in the configuration.

Fix invalid JSON in Builds API doc

See merge request !7475

Add ref parameter for triggering builds with gitlab webhook from other project.

Currently GitLab can trigger builds from external tools like curl.
But there is no way to trigger builds with webhook from another GitLab project.
Executing webhook like `/projects/:id/trigger/builds?token=TOKEN&ref=master` results in such message from server:
```
400 No builds created
```

Problem is the ref from request body takes precedence of the ref from query string.
System tries to trigger build for nonexistent branch instead of `master`.
This is the feature of the Grape framework used to build GitLab api.

This MR adds optional `ref` parameter into the api url:
```
/projects/:id/ref/:ref/trigger/builds?token=TOKEN
```

This will lead that the ref in the url will have precedence of the ref inside request body.

This solution provides compatibility with current API urls.

Closes #23584

See merge request !7022