Add a proper About section

Created by: riyad

What should be on there?

• Some basic information about that particular installation (GitLab version, Gitolite version, admin email, ...)
• Information on GitLab as a project (project website, code, license, support channels, ...)
• Listing of the help pages

Where to place it?

• It can easily replace the "Help" section on the Dashboard
• Add it to the title bar?
• Add a footer?