Fix a 500 error when the `restricted_visibility_levels` setting is null
in the database.

This adds the functionality of replacing all images that were uploaded
to gitlab with inline images(base64) in emails.
This change fixes the broken images in emails that 7.8 introduced

Bug fixes and new tests for the restricted visibility changes.

Also preserve code preview color scheme in events dashboard.

Assign default colors to all code blocks shown as <pre class="code highlight [color_scheme]">

Closes #1139

Add new service classes to create and update project and personal
snippets.  These classes are responsible for enforcing restricted
visibility settings for non-admin users.

Add checkboxes to the application settings page for restricted
visibility levels, and remove those settings from gitlab.yml.

Supports four different event types all bundled under the "note" event type:

- comments on a commit
- comments on an issue
- comments on a merge request
- comments on a code snippet

Closes #1151

* Impl Project#avatar_url
* Refactor ApplicationHelper: Use Project#avatar_url
* Update changelog

Added a background, rather than an outline, which should reduce
clutter on the screen.

The filter icons (Push events, Merge events, Comments, Team) now have a border
that signifies they can be pushed. This not only keeps it in line with the rest
of the application buttons, but it makes it more obvious, especially for the
Merge events button with a checkbox icon.

Tooltips now have meaning by mentioning their function ("Filter by..."). Fixes #1992

Make the following changes to deal with new behavior in Rails 4.1.2:

* Use nested resources to avoid slashes in arguments to path helpers.

Example: "User joined project Namespace / Project" rather than "User joined project at Namespace / Project"