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