Added KaTeX license and procedure to build it for Gitlab

## What does this MR do?
Adds KaTeX license and the procedure for building KaTeX for gitlab. 

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

## Why was this MR needed?
Requested by @stanhu

## Screenshots (if relevant)
Not relevant. 

## Does this MR meet the acceptance criteria?

- [X] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) 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] 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?

See merge request !8132

Mattermost Notifications Service

## What does this MR do?
closes #22269 

## Screenshots 

![mattermost](/uploads/de71c121f544a91305b6dfa6dc4c5738/mattermost.png)  
![slack](/uploads/081d75d49239319d94332abda214fb98/slack.png)

## Does this MR meet the acceptance criteria?

- [x] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) 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] 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)


See merge request !7764

Fix 8.15 RC1 requires quite amount of downtime

See https://gitlab.com/gitlab-org/gitlab-ce/issues/25635

See merge request !8106

Update grape to 0.18.0

## What does this MR do?

Update grape to 0.18.0. Required for the settings API.

## What are the relevant issue numbers?

#22928

See merge request !8057

Fix specs in Ruby 2.1

Ruby 2.1 requires a basename argument to `Tempfile.open`, so just call
it something that makes sense in context for the spec.

See merge request !8138

Replace static fixture for abuse_reports_spec

## What does this MR do?

Replace static HAML fixture for `abuse_reports_spec` by dynamically created one.

## What are the relevant issue numbers?

#24753

See merge request !7644

Add missing group policy spec

See merge request !8122

Made Ci::Builds to have same ref as Ci::Pipeline in dev fixtures

See merge request !8110

Update custom hooks docs and set 4.1.0

See merge request !8040

Ruby 2.1 requires a basename argument to `Tempfile.open`, so just call
it something that makes sense in context for the spec.

UX Guide: add guidance on cursor usage

Add guidance to the UX guide on when to use what mouse cursor.

See merge request !8128

UI improvements for nested group feature

See merge request !8062

Fix eslint errors

Fixes current eslint errors on master

See merge request !8127

Fix spec/features/admin/admin_active_tab_spec.rb

See merge request !8130

Don't open Asciidoc module twice

See merge request !8119

Add Okta authentication documentation

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/14122

See merge request !8104

Add `gitlab_rails['auto_migrate'] = false` to HA docs for Redis/PG [ci skip]

## What does this MR do?

In a high availability configuration, the Redis and PostgreSQL nodes
should not attempt to run database migrations. In fact, trying will
result in errors about not providing a database password. To
prevent errors and confusion, add this configurtion to these nodes'
`gitlab.rb` file.

See merge request !8112

Merge branch '25144-gitlab-ce-mattermost-slash-command-for-issue-create-needs-better-documentation' into 'master'

Resolve "gitlab-ce mattermost slash command for issue create needs better documentation"

## What does this MR do?

Updates the documentation and the <kbd>help</kbd> command to be clearer, having the keys used to add a newline in chat clients (both Mattermost and Slack). 

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

* Are the available commands (via <kbd>help</kbd>) being formatted as `<code>` ?

## Why was this MR needed?

`\n` represents a new line character and doesn't communicate how the user should input the command. Also, to be correct, the documentation should use `<kbd>` instead of `<code>` for user input ([see HTML5 specification](https://www.w3.org/TR/html5/text-level-semantics.html#the-kbd-element))

## Screenshots (if relevant)

| Mattermost | Docs |
| --- | --- |
| ![image](/uploads/539526a14bfd551b7e732dd96c5b7581/image.png) | ![image](/uploads/e3eb099f86c5a32ce3b8954e72c29848/image.png) |

## Does this MR meet the acceptance criteria?

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

## What are the relevant issue numbers?

Closes #25144

See merge request !7850

Improve bulk assignment

This MR improves current implementation of Label dropdown when used for bulk assignment on issuable pages (/:namespace/:project/issues, /:namespace/:project/merge_requests)

Previously this dropdown relied on `<input>` tags to get its active items and also to calculate items with indeterminate state.

Relying on `<input>` tags is not enough when we want to set/get multiple states on a dropdown.

For this case we want to get/set:

- Marked items 
- Unmarked items that were initially marked
- Unmarked items that were initially indeterminate
- Items with indeterminate state.

This MR makes the Label dropdown to save its own state as `data` so it will be easy to get and set whatever state we want no matter if the dropdown is filtering which is the issue that I initially wanted to solve as you can see in the following gif.

**Before** 
![2016-12-07_11.44.48](/uploads/cb697161b8b39cdee72fdbb95a531100/2016-12-07_11.44.48.gif)

**After**
![2016-12-07_11.32.43](/uploads/338255a302de0dd1367474f33232d2a3/2016-12-07_11.32.43.gif)

As you can see in the first gif the `bug` label is removed from the selected issues but the `enhancement` label should set but the `critical` should be kept. This is fixed on the next gif.

Fixes #24877

See merge request !7765

25617 Fix placeholder color of todo filters

## What does this MR do?
Fixes color of placeholder text on todo filters

## Screenshots (if relevant)

![Screen_Shot_2016-12-14_at_1.42.02_PM](/uploads/7ceec3c8fff47825384835bcfb38d77c/Screen_Shot_2016-12-14_at_1.42.02_PM.png)

## 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
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] 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?
Closes #25617

See merge request !8094

24824 Add focus state to dropdowns

## What does this MR do?
Adds focus state to dropdown items to increase accessibility. 

## Screenshots (if relevant)

![Screen_Shot_2016-12-01_at_2.47.13_PM](/uploads/d0d2c17dde61314c86155268815091f6/Screen_Shot_2016-12-01_at_2.47.13_PM.png)

## 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
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] 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?
Closes #24824

See merge request !7885

Show commit status from latest pipeline

Show commit status from latest pipeline rather than compound status from all pipelines.

Closes #20560

See merge request !7333

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Remove unnecessary commits order message

Remove unnecessary message. User should expect that commits are listed in reverse chronological order. Even if they are not initially clear, glancing quickly through the list informs them right away. No point in drawing unnecessary attention to extra information that is not main content. The other tabs also have lists in chronological or reverse chronological order. But there's nothing there to explain in which direction.

### Existing
<img src="/uploads/d5592533c60186a7a53af5d6d010fa56/Screen_Shot_2016-12-09_at_12.38.25_AM.png" height="200px" />

### After change
<img src="/uploads/facc800496d27f1ee33e0d48d339b2f3/Screen_Shot_2016-12-09_at_12.37.36_AM.png" height="200px" />

See merge request !8004

Fixed GFM autocomplete to disallow non-word characters in string

## What does this MR do?

Disallows GFM autocomplete working when a non-word character is enter in the middle of the string.

## Screenshots (if relevant)

![Screen_Shot_2016-12-12_at_09.47.46](/uploads/aa6463a57b97c103c69eab6bb1d80540/Screen_Shot_2016-12-12_at_09.47.46.png)

## What are the relevant issue numbers?

Closes #25540

See merge request !8035

added go back anchor on error pages

## What does this MR do?

adds go back anchor on error pages

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

## Why was this MR needed?

requested per https://gitlab.com/gitlab-org/gitlab-ce/issues/25534

## Screenshots (if relevant)

![image](/uploads/894c513e7a058deb0124b5152dc11e28/image.png)

## Does this MR meet the acceptance criteria?

- [x] [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
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] 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?

Closes #25534

See merge request !8087

Add Object.assign polyfill

Because PhantomJS

See merge request !8090