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: NDmitriy 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

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

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

[ci skip]

Refactor commits API documentation

Blocked by https://gitlab.com/gitlab-org/gitlab-ce/issues/10960 which needs to be fixed or documented.

Closes #4217 

See merge request !2465

[ci skip]

Fix a missing letter in the documentation.

Hello,

I just found a missing letter in the PHP CI configuration documentation. This MR allow to fix that 😄 

See merge request !2519

Fix MR diff 'Edit' button

It now works and has the right size.

See merge request !2542

More highlighted diff fixes

See merge request !2545

Fix grammar and typos in file finder docs



See merge request !2547

[ci skip]

Make sure non-highlighted diffs are still escaped

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/12521

See merge request !2544

File finder can be invoked from anywhere!



See merge request !2543

[ci skip]

Add method to calculate total size of artifacts in subpath

See merge request !2500

Diff highlight themes

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/12484

See merge request !2530