Add style for <kbd> element in markdown

Fixes #5825

Use slightly fewer icons in project sidebar.

![Screen_Shot_2015-05-14_at_17.01.37](https://gitlab.com/gitlab-org/gitlab-ce/uploads/57928b7a0beb556f1e779d340b5e124f/Screen_Shot_2015-05-14_at_17.01.37.png)

See merge request !657

Minor RelativeLinkFilter cleanup

See merge request !649

Protect OmniAuth request phase against CSRF.

Addresses #2268.

See merge request !1793

Fix mentioning of private groups.

`issue.participants` would mistakenly not send the `current_user` to `mentioned_users`, which means it would search for referenced groups and users as if the user was signed out. Since signed out users don't have access to private groups (groups without public projects) and since you can't reference a group you don't have access to (since this would leak its existence), the group reference wasn't found and the group members weren't notified.

Addresses internal issue https://dev.gitlab.org/gitlab/gitlabhq/issues/2326.

See merge request !654

Always show note action icons.

As discussed in #639.

The delete icon becomes red on hover, the edit one becomes slightly darker.

The "Link here" icon has been removed, and the link to the specific comment can now be found by clicking the post date ("3 months ago"), which is most other sites do it as well.

![Screen_Shot_2015-05-14_at_12.18.50](https://gitlab.com/gitlab-org/gitlab-ce/uploads/a204fa3a042659fa53fcb70d7143c8d1/Screen_Shot_2015-05-14_at_12.18.50.png)

See merge request !652

Use apt repositories to upgrade Docker images to the latest versions

Reduces the need to bump versions periodically.

See merge request !647

Reduces the need to bump versions periodically.

Use SIGKILL by default in Sidekiq::MemoryKiller

This makes the memory growth-triggered Sidekiq restarts more reliable by
reducing the chance that Sidekiq ends up in a state where it is not
accepting new jobs but also not shutting down: SIGKILL is more likely to
work than SIGTERM.

See merge request !1824

This makes the memory growth-triggered Sidekiq restarts more reliable by
reducing the chance that Sidekiq ends up in a state where it is not
accepting new jobs but also not shutting down: SIGKILL is more likely to
work than SIGTERM.

Batch 1 of text improvements

Batch 1 of changes from my effort at !635 to walk through every piece of text in GitLab and see if it can be improved.

This batch includes:

- Improve text on error pages.
- Improve Git access error messages.
- Improve description of branch protection levels.
- Improve OAuth signup error message.
- Improve OAuth application flash messages.

cc @rspeicher

See merge request !642

Version check

See merge request !1509

Extract handling of relative file links to its own HTML filter

Call merge request web hook for each commits

defreeze code

Fixes #2166.

See merge request !1820