Move reply by email docs to a new location

## What does this MR do?

Move reply by email docs to a new location.

Part of https://gitlab.com/gitlab-org/gitlab-ce/issues/3349

## Moving docs to a new location?

See the guidelines: http://docs.gitlab.com/ce/development/doc_styleguide.html#changing-document-location

- [ ] Make sure the old link is not removed and has its contents replaced with a link to the new location.
- [ ] Make sure internal links pointing to the document in question are not broken.
- [ ] Search and replace any links referring to old docs in GitLab Rails app, specifically under the `app/views/` directory.
- [ ] If working on CE, submit an MR to EE with the changes as well.

See merge request !6517

Update pipeline graph styles to match mockup

#### What does this MR do?
* Updates CSS for pipeline graph to match mockup 
* Adds JS to dynamically change line widths based on number of builds

#### Screenshots (if relevant)
![Screen_Shot_2016-10-04_at_2.06.24_PM](/uploads/362378ca23e18a3cc59185eafab9f9ac/Screen_Shot_2016-10-04_at_2.06.24_PM.png)

#### What are the relevant issue numbers?
Closes #21604

See merge request !6320

Add SubGit to the tools of SVN migration

Originally from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6549.

See merge request !6803

[ci skip]

Document the new CI_DEBUG_TRACE variable a feature that will
be introduced in GitLab Runner v1.7

Related to https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/merge_requests/339

See merge request !6700

[ci skip]

Fix Error 500 when viewing old merge requests with bad diff data

Customers running old versions of GitLab may have MergeRequestDiffs with the text ["--broken diff"] due to text generated by gitlab_git 1.0.3.
To avoid the Error 500, verify that each element is a type that gitlab_git will accept before attempting to create a DiffCollection.
    
Closes #20776

See merge request !6754

Add a new gitlab:users:clear_all_authentication_tokens task

## What are the relevant issue numbers?

Part of #22537.

See merge request !6745

Update mail_room gem

## What does this MR do?

Upgrades the mail_room gem from 0.8.0 to 0.8.1.

## Why was this MR needed?

mail_room throws an exception if the watching script joins a undefined thread. Release 0.8.1 adds a check to ensure that the thread exists before calling join.

Fixes gitlab-org/omnibus-gitlab#1619.

See merge request !6714

Remove redundant images from GitLab basics docs



See merge request !6799

Add an example for testing a phoenix application with Gitlab CI

_Originally opened at !5074 by @nahtnam._

- - -

## What does this MR do?

Add an example on how to setup testing on phoenix 

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

Grammar and formatting

## Why was this MR needed?

https://twitter.com/gitlab/status/749898132008337408

## 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)
- Tests
  - [ ] All builds are passing
- [ ] 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 you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !6794

Replace images in GitLab basics docs



See merge request !6786

[ci skip]

Precalculate trending projects

## What does this MR do?

This MR changes the trending projects code so that data is precalculated, removing the need for any complex caching mechanisms. 

## Why was this MR needed?

Caching of trending data didn't work properly, still leading to queries. Using caching in general would be very hard due to users being able to apply custom filters to the list of trending projects.

See merge request !6749

Allows empty merge requests for a workflow described in gitlab-org/gitlab-ce#14606

Closes gitlab-org/gitlab-ce#14606

See merge request !6384

[ci skip]

Add `robots.txt` to the list of reserved namespaces

See #22759

See merge request !6789

Bump mail_room to v0.8.1 to fix thread cleanup issue

See https://github.com/tpitale/mail_room/blob/master/CHANGELOG.md

Closes #20273

See merge request !6788