Cache project avatars stored in Git

Related issue: https://gitlab.com/gitlab-org/gitlab-ce/issues/14363

See merge request !3272

The avatar logic has been moved from Project to Repository as this makes
caching easier. The logic itself in turn has been changed so that the
logo file names are cached in Redis. This cache is flushed upon pushing
a commit but _only_ if:

1. The commit was pushed to the default branch
2. The commit actually changes any of the logo files

If no branch or commit is given the cache is flushed anyway, this
ensures that calling Repository#expire_cache without any arguments still
flushes the avatar cache (e.g. this is used when removing a project).

Fixes gitlab-org/gitlab-ce#14363

Pre-selects values on issue and merge request dropdown

Closes #14290

See merge request !3233

Adjusted behavior so canceled builds tagged as allowed to fail do not fail build

## What does this MR do?
This diff changes the 'Allowed to fail' flag to also ignore canceled builds, whereas before, canceled builds could fail the suite even if they were marked as 'Allowed to fail'

dupe of !3258 to force a build

## Are there points in the code the reviewer needs to double check?
no

## Why was this MR needed?
Unexpected behavior as a user

## What are the relevant issue numbers?

## Screenshots (if relevant)
![whatisgoingon](/uploads/fcd7c8f3d8454bc730d7fd41eff59b31/whatisgoingon.png)

See merge request !3271

Merge duplicate cache entry in CI docs



See merge request !3208

PostReceive refactor

Back-ported PostReceive refactor made for https://gitlab.com/gitlab-org/gitlab-ee/issues/367 in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/267

See merge request !3264

External Users

The user has the rights of a public user execpt it can never create a project, group, or team. Also it cant view internal projects.

Partially fix for both #4009 and #13938 (except the LDAP sync or a pattern to detect external users)


![Screen_Shot_2016-03-14_at_22.02.52](/uploads/486a84ab3acb98c6cfb71a4ec7d268e2/Screen_Shot_2016-03-14_at_22.02.52.png)

See merge request !3171

New branch button on issues

Creates a button which creates a branch for you, referencing the original issue in the branch name. When creating the MR the text `Closes #iid` is appended to the description field.

The button; with styling

![Screenshot_from_2016-02-13_17-28-54](/uploads/9595fbfef88c10a499829beaa3db11c1/Screenshot_from_2016-02-13_17-28-54.png)

Links to the related branches on the issue

![Screenshot_from_2016-02-13_17-31-48](/uploads/ee7a78754eb5e2389f8671771bb59af9/Screenshot_from_2016-02-13_17-31-48.png)

Styled like the MR links

![Screenshot_from_2016-02-13_17-31-53](/uploads/c60130333a650a16f51b014aeb1458c3/Screenshot_from_2016-02-13_17-31-53.png)


Please provide input on the following; the CI repo on the GDK had the following happening with the current way I implemented the matching:

![Screenshot_from_2016-02-14_08-57-19](/uploads/35e7a3687a0fa801aba1ad66305ab2ff/Screenshot_from_2016-02-14_08-57-19.png)

Closes #3886 

cc @DouweM 


See merge request !2808

Fix horizontal overflow that occurs in the code tag

closes #14167 

### Before Fix
![Screen_Shot_2016-03-16_at_9.10.52_PM](/uploads/b4a282ffc33684983d7e2482b8e13a8f/Screen_Shot_2016-03-16_at_9.10.52_PM.png)

### After Fix
![Screen_Shot_2016-03-16_at_9.11.16_PM](/uploads/8acca3b46bfdbd929cc096ad070022e2/Screen_Shot_2016-03-16_at_9.11.16_PM.png)


See merge request !3255

Follow the CSS Style Guide rules for including a space after the property colon.

Fixes violations of SpaceAfterPropertyColon.

Discussed in #14299.

[ci skip]

cc: @rspeicher @jschatz1 

See merge request !3260

Reuse `no_cache_headers` method in badges controller

Closes #14100

See merge request !3269

Spinach was right, I was a fool..

[ci skip]

Fix typo in README.md



See merge request !3250

[ci skip]

Fix wiki search results point to raw source

Closes https://github.com/gitlabhq/gitlabhq/issues/9581

See merge request !3138

Fixes violations of SpaceAfterPropertyColon.

Discussed in #14299.

[ci skip]

Follow the CSS Style Guide rules for unitless zero values.

Discussed in #14299.

cc: @rspeicher @jschatz1 

See merge request !3259

Updated Emoji Rake task to conform to style guide.

Discussed in #14299.

[ci skip]

omniauth-auth0 gem support

See merge request !3246

Fixes issue with dropdown caret being incorrect color

Previously:

![Screen_Shot_2016-03-16_at_19.31.20](/uploads/b887a7b8249f172513c68289c52a4cf7/Screen_Shot_2016-03-16_at_19.31.20.png)

Now:

![Screen_Shot_2016-03-16_at_19.29.37](/uploads/9c0d6a3002c18a8c7890df8dce612dcb/Screen_Shot_2016-03-16_at_19.29.37.png)



See merge request !3254

See merge request !3249

Follow the CSS Style Guide rules for HEX colors.

Use lowercase letters and shorthand values for HEX colors where possible, in accordance with the CSS Style Guide.

Discussed in #14299.

\cc @jschatz1 @rspeicher

See merge request !3238

Install Git 2.7.3, not 2.4.3



See merge request !3248