Add `visibility` & `visibility=` methods to the
`Gitlab::VisibilityLevel` module so the `visibility_level` can be
get/set with a string value.

For the API, the VisibilityLevel will be exposed as String instead of
Integer. So add the string values and method to translate a level
integer to a string.

This reverts commit 3e991230.

# Conflicts:
#	app/models/project.rb

Prevent Groups to have smaller visibility than projects
Add default_group_visibility_level to configuration
Code improvements

Return `true` from `non_restricted_level?` when the
`restricted_visibility_levels` setting is nil.

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

Signed-off-by: Jeroen van Baarsen <jeroenvanbaarsen@gmail.com>

Added visibility_level icons to project view (rather than just text).
Added public projects to search results.
Added ability to restrict visibility levels standard users can set.