Remove emphasis from system notes to avoid Markdown conflicts in names.

This reverts commit 07f9a3f9, reversing
changes made to 48036751.

Reverted this because the data is used in hooks as well.

Remove unnecessary fetch of commit messages for initial push. This will reduce the memory usage significantly.

It seems that ruby can handle 'nil' value but other json processors
(like PHP) throw an error. This is always generated for empty arrays.

This commit disables process_commit_messages()
for the initial push to the default branch.
This fixes the mass SQL statements (~500000) that were executed during
the initial push of the linux kernel for example.

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.

Allow admins to use restricted visibility levels when creating or
updating projects.

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