Move 'Explore Snippets' Spinach feature to Rspec

## What does this MR do?
It moves the `features/snippets/discover.feature` Spinach test to a Rspec feature.

## Are there points in the code the reviewer needs to double check?
The original feature was called 'Discover Snippets', but the UI no longer reflects this wording. The new Rspec feature is called 'Explore Snippets' to reflect UI/Controller/View naming in use.

## Why was this MR needed?
As part of deprecating the Spinach test suite.

## Screenshots (if relevant)

## Does this MR meet the acceptance criteria?

- [-] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) added
- [-] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [-] API support added
- Tests
  - [x] Added for this feature/bug
  - [ ] All builds are passing
- [x] 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?
#23036

See merge request !7349

Do not create a MergeRequestDiff record when source branch is deleted

Fixes #23413 
Fixes #23897

See merge request !7481

Limit labels returned for a specific project as an administrator

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/24527

See merge request !7496

Use setter for key instead AR callback

## What does this MR do?

- [x] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) added
- n/a [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- n/a API support added
- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [x] 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)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6763

See merge request !7488

fix labels API adding missing parameter (current_user)

See merge request !7458

Fix a badly-performing migration

See merge request !7483

Move 'Search Snippets' Spinach feature to Rspec

## What does this MR do?
Moves the Spinach feature `features/snippet_search.feature` to an RSpec feature `spec/features/snippets/search_snippets_spec.rb`.

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

The search results page URL was being visited directly in the original Spinach test, using an encoded search query and specific parameters to trigger the snippet searches. I have changed the RSpect feature to fill in the search box on the snippets dashboard page to retrieve results to cover the missing test coverage on the search box and to avoid the feature test from relying on the implementation details of the search URL structure.

## Why was this MR needed?

As part of deprecating the Spinach test suite.

## Screenshots (if relevant)

## Does this MR meet the acceptance criteria?

- [-] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) added
- [-] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [-] API support added
- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [x] 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?

#23036

See merge request !7494

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

Prior, an administrator viewing a project's Labels page would see _all_
labels from every project they had access to, rather than only the
labels of that specific project (if any).

This was not an information disclosure, as admins have access to
everything, but it was a performance issue.

[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]

In order to have a valid MergeRequestDiff record when need head_commit_sha.
When a source branch is deleted head_commit_sha is nil.
This causes an exception in merge request "Changes" tab.

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>

This commit moves the `search_snippets.feature` Spinach test to a
Rspec feature, as part of deprecating the Spinach test suite.

- Remove Spinach discover snippets feature and steps
- Remove unused `SharedSearch` module
- Add Rspec feature scenarios

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