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