Shorten line length of issues and mrs

Limits container width on issue and mr pages. Shortens line length to human readable length.

![Screen_Shot_2016-12-02_at_12.48.08_PM](https://gitlab.com/gitlab-org/gitlab-ce/uploads/740647c0fd6ea4f0f0aef62155992afe/Screen_Shot_2016-12-05_at_12.54.12_PM.png)

Closes #20916 
Closes #24408

See merge request !7858

Add GitLab host to 2FA QR and manual info

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/25545

See merge request !6941

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

The two factor authentication account string only had the user's
email address. This led to ambiguous entries in two factor
code generating apps. This adds the GitLab host to the account
string in the standard format (according to Google). No matter
the code generator this change disambiguates the entry.

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