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]

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]

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

This ensures that an instrumented method that doesn't take arguments
reports an arity of 0, instead of -1.

If Ruby had a proper method for finding out the required arguments of a
method (e.g. Method#required_arguments) this would not have been an
issue. Sadly the only two methods we have are Method#parameters and
Method#arity, and both are equally painful to use.

Fixes gitlab-org/gitlab-ce#12450

[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.