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]

Add references to the rouge gem library

Give the props to this awesome library we use
for syntax highlighting.

See merge request !2605

Prioritize previewable over plain README files

Fixes #12441

See merge request !2521

[ci skip]

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

[ci skip]

Substituted deprecated forum link with project issues link.

Within the promo partial template we had a Requests link to the
deprecated Forum which is now directing to the project's Issue page.

See merge request !2255

[ci skip]

- Add GitLab configuration steps
- Add example workflow
- Replace old images with new ones

Adds base64 background search icon.

All inputs of type `search` will have the gray background
and search icon centered.
Because the search magnifier is a bg image,
I had to hide it with `[value=""]`.
I added a little javascript to make sure each input always has it's
own `value`.

A text and icon placeholder in the center of the input that disappears when typed is only possible with javascript. If you just want the icon or just the text it can be done with css alone.
If you want the icon/text justified left, then the search magnifier does not have to disappear. 

Fixes #11870 

cc @skyruler @creamzy 

![searchinput](/uploads/136ab91d4ff9ce717979d0ce1a23ab03/searchinput.gif)


See merge request !2546

[ci skip]

Rails updated to 4.2.5

I hope it'll fix weird spec failing (according to https://github.com/rails/rails/pull/22383)

@dzaporozhets @yorickpeterse 

See merge request !2591

Install unzip in build environment [ci skip]



See merge request !2593

Update gitlab_git & use new method for counting branches

Corresponding gitlab_git merge request detailing some of the rationale behind this: https://gitlab.com/gitlab-org/gitlab_git/merge_requests/62

Fixes #12418 

See merge request !2535

[ci skip]

Only top search is gray.

Main search input has different styles than secondary search inputs.

All inputs of type `search` will have the gray background
and search icon centered.
Because the search magnifier is a bg image,
I had to hide it with `[value=""]`.
I added a little javascript to make sure each input always has it's
own value.

Add note on artifacts browser functionality

Based on the following comment:
https://gitlab.com/gitlab-org/gitlab-ce/issues/12634#note_3359006

See merge request !2586

Based on the following comment:
https://gitlab.com/gitlab-org/gitlab-ce/issues/12634#note_3359006

[ci skip]

Move supported omniauth providers to the top



See merge request !2585

[ci skip]

Move all integration docs images in a single directory



See merge request !2584

[ci skip]

Refactor system_hooks API documentation



See merge request !2475

Refactor namespaces API documentation



See merge request !2472

Refactor README API documentation

~~Waiting for https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/2435 to be merged.~~

See merge request !2476

[ci skip]