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

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Minor spelling, prob typo



See merge request !2564

Name more OmniAuth providers in the general readme.



See merge request !2566

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Don't use production section for Gemfile

Related to !2485

See merge request !2570

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Speed-up docker based builds

This adds:
- Bundler cache
- APT cache
- PhantomJS cache

It should reduce the time needed for container to prepare docker-based build environment.


See merge request !2567

Provide a link for fetching merge requests from command line instructions

Closes #2239 

See merge request !2540

Make deleted line background color for Solarized Dark a little darker

The difference is very slight, and I don't even _use_ Solarized Dark, but this annoyed me.

See merge request !2563

Backport some changes from EE



See merge request !2548

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Fix Error 500 when doing a search for merge requests

Potential fix for #11547

See merge request !2525

Enable to:
- Use APT cache
- Use Bundler cache
- Use PhantomJS cache

Leaving out the -I option in "git grep" would cause an Error 500 because the
resulting line would include "Binary file X matches"

Allow empty Mysql password on CI

[ci skip]

See merge request !2558

GitLab 8.4 uses gitlab-shell v2.6.10

While upgrading to GitLab 8.3 to 8.4 there was a warning massage for using GitLab 8.3 with gitlab-shell v2.6.9 so I changed gitlab-shell version to v2.6.10 and it worked.

See merge request !2555

[ci skip]

Fix broken codeblocks in ci/yaml documentation



See merge request !2554

[ci skip]

Remove 8.4 unreleased note from changelog



See merge request !2553

[ci skip]

Fix wording and appearance on build artifacts documentation



See merge request !2550

[ci skip]

Add image to 2fa security documentation



See merge request !2549