Improved redis sentinel documentation

## What does this MR do?
Updates Redis Sentinel documentation (most Omnibus stuff).

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

## Why was this MR needed?
After this https://gitlab.com/gitlab-org/omnibus-gitlab/merge_requests/1000 get merged, people will need to follow new instructions.

## Checklist

- Documentation follows Sentinel best-practices:
  - [x] Suggests amount of sentinel nodes
  - [x] Suggests amount of redis nodes
  - [x] Suggests quorum value according to recomended amount of sentinel nodes
  - [x] Describes how to define quorum according to best-practices
  - [x] Lists ports and firewall checklist
- [x] Following Documentation bring us to a working environment
- [ ] Documentation is clear and have no gramatical issue

## What are the relevant issue numbers?

gitlab-org/omnibus-gitlab#1565

See merge request !6471

[ci skip]

[ci skip]

Navigation bar issuables counters reflects dashboard issuables counters

Adds a 2 minute cache on navigation bar issuables counters.

The counters on dashboard/issues and dashboard/merge_requests are cached while the navigation bar issuables counters were not. Users read incoherent issuables count throughout the application.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/23882

See merge request !7368

Remove gitattribute entry for CHANGELOG.md

This should no longer be necessary now that we've got automatic changelogs!

See merge request !7479

Limit autocomplete to currently selected items

When using the /unlabel command the autocomplete list of labels is now
limited to the list of labels currently assigned to the issue.

Closes #22680

See merge request !6796

[ci skip]

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

[ci skip]

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