Prioritize previewable over plain README files

Fixes #12441

See merge request !2521

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

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

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

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]

Refactor settings API documentation



See merge request !2474

[ci skip]

Refactor labels API documentation



See merge request !2471

Refactor issues API documentation



See merge request !2466

Refactor deploy_keys API documentation



See merge request !2467

Refactor branches API documentation



See merge request !2464

Don't vendor minified JS

Fixes #4031

See merge request !2494

Whitelist raw "abbr" elements when parsing Markdown

See #12517.

See merge request !2559

Forcefully install apt packages



See merge request !2577

Closes #12517

Ignore binary files in code search to prevent Error 500

Leaving out the -I option in "git grep" would cause an Error 500 because the
resulting line would include "Binary file X matches", which would cause the project search results to crash with the following error:

```
ActionView::Template::Error (wrong argument type nil (expected Regexp)):
    1: - blob = @project.repository.parse_search_result(blob)
    2: .blob-result
    3:   .file-holder
    4:     .file-title
  app/models/repository.rb:742:in `sub'
  app/models/repository.rb:742:in `block in parse_search_result_from_grep'
  app/models/repository.rb:741:in `each_line'
  app/models/repository.rb:741:in `parse_search_result_from_grep'
  app/models/repository.rb:682:in `parse_search_result'
  app/views/search/results/_blob.html.haml:1:in `_app_views_search_results__blob_html_haml__1959871337755590783_162450160'
  app/views/search/_results.html.haml:20:in `_app_views_search__results_html_haml__2198486911700532411_58329920'
  app/views/search/show.html.haml:7:in `_app_views_search_show_html_haml___1698304427272645201_62235300'
```

See merge request !2565

Make git gc man page more apparent



See merge request !2557

Remove gray background from issue/mr/milestone background

Fixes #6062 
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
cc @skyruler @creamzy 

See merge request !2573

Fix UI for pages with panels
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

cc @skyruler @creamzy 

Fixes #12633 

See merge request !2571