Update ExternalIssue regex for JIRA integration

The pattern in the `::reference_pattern` class method in the
`ExternalIssue` model does not match all valid forms of JIRA project
names. I have updated the regex to match JIRA project names with numbers
and underscores. More information on valid JIRA project names can be
found here:
https://confluence.atlassian.com/jira/changing-the-project-key-format-192534.html

* The first character must be a letter,
* All letters used in the project key must be from the Modern Roman Alphabet and upper case, and
* Only letters, numbers or the underscore character can be used.

See merge request !2598

The pattern in the `::reference_pattern` class method in the
ExternalIssue model does not match all valid forms of JIRA project
names. I have updated the regex to match JIRA project names with numbers
and underscores. More information on valid JIRA project names can be
found here:
https://confluence.atlassian.com/jira/changing-the-project-key-format-192534.html

* The first character must be a letter,
* All letters used in the project key must be from the Modern Roman Alphabet and upper case, and
* Only letters, numbers or the underscore character can be used.

Bug/jschatz1/file browser lines

Fixes #12725 

![Screen_Shot_2016-01-26_at_11.19.39_AM](/uploads/c5f5a143926600e02de4cfc0687ed8dd/Screen_Shot_2016-01-26_at_11.19.39_AM.png)


See merge request !2615

Bump Workhorse version to 0.6.2

Fix for #12634 has been introduced in gitlab-org/gitlab-workhorse!34.

See merge request !2616

Merge branch 'search-box-in-group-members-view-is-too-small-for-placeholder-in-chrome' into 'master'

Fixes: Search box in Group Members view is too small for placeholder in Chrome

Fixes #2498 

![Screen_Shot_2016-01-25_at_11.55.57_PM](/uploads/de4fda98cd8552641208b3a7a9277e35/Screen_Shot_2016-01-25_at_11.55.57_PM.png)


See merge request !2609

This has been fixed in Workhorse [ci skip]

Fix for #12634 has been introduced in gitlab-org/gitlab-workhorse!34.

Monkey patch for mysql 5.7



See merge request !2597

Uses `$border-color`

Clicks the edit button instead of opening the dropdown

Fixes #2307 

Instead of opening the menu, this clicks the `edit` button.

See merge request !2607

This reverts commit a1408948.

This reverts commit 11183488.

Fixes #12725

Fix visibility level texts on application settings

Introduced by me through !2005

Before:

![Screenshot_from_2016-01-26_09-34-09](/uploads/fa3e8f2522a1d26372c7a06a81119e6f/Screenshot_from_2016-01-26_09-34-09.png)

After:

![Screenshot_from_2016-01-26_09-34-27](/uploads/7955003dd02dd5376c9966aafdf4c50b/Screenshot_from_2016-01-26_09-34-27.png)


See merge request !2611

Introduced by me through !2005

New download btn WIP.

Save button on app settings now btn-save

Some save buttons where `btn-primary` where throughout the application these usually are `btn-save`.

Before (Application settings)

![Screenshot_from_2016-01-26_09-44-32](/uploads/f4019349c8bac6e0c91e23252a70d334/Screenshot_from_2016-01-26_09-44-32.png)

After

![Screenshot_from_2016-01-26_09-45-20](/uploads/8e59a2b180832f44e3908b22187549af/Screenshot_from_2016-01-26_09-45-20.png)


See merge request !2612

Track project import failure

Fixes #12512 

This also help us on identify why the import process is returning the wrong status when the project was successfully imported. Like mentioned in the issue #12450 

See merge request !2538

Add tests for clicking a row in build artifacts browser

This adds a feature tests for clicking a row, which is handled by javascript event.

/cc @ayufan 

See merge request !2551

Update text_color_for_bg helper to support RGB triplet color codes

Closes #12677

See merge request !2587

Make the `/groups` route behave as expected

The route is supposed to redirect the Groups#index request based on
whether or not a user was logged in. If they are, we redirect them to
their groups dashboard; if they're not, we redirect them to the public
Explore page.

But due to overly aggressive `before_action`s that weren't excluding (or including) the
`index` action, the request always resulted in a 404, whether a user was
logged in or not.

Closes #12660

See merge request !2580

Update Slack integration configuration

Adjusted to the current Slack version (as of I 2016).

See merge request !2595

[ci skip]

Fixes #2498

Fixes #2307

Fix import redirect loop

Fixes #11864 

See merge request !2606

Correct arity for instrumented methods w/o args

This should fix the problem of imports not working. See https://gitlab.com/gitlab-org/gitlab-ce/issues/12450#note_3380730 for the details/context of these changes.

See merge request !2602

Highlight note code and edit preview

More highlighting fixes! Highlighting wasn't applied all the way for
code in notes, diff notes diffs and diff preview (on the file edit page).

The CSS changes further unify the styling for file-content highlighting,
diff highlighting and note code highlighting.

See merge request !2594

Update rails, rails-html-sanitizer, and nokogiri for security fixes

See https://dev.gitlab.org/gitlab/gitlabhq/issues/2643

See merge request !2603

Refactor JIRA documentation

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/12680

## TODO

- [x] Move JIRA docs from `integration/` to `project_services/`
- [x] Split old JIRA info
- [x] Add note on document migration
- [x] Change references to new location
- [x] Add new images
- [x] Clean-up `jira.md`

See merge request !2592

[ci skip]