Add page that lists applications that support GitLab

proposed URL: /applications

link to it from the /gitlab-ce page

state on the page that apps can add themselves by sending an MR

Fill with:

• Usersnap
• Drupal https://www.drupal.org/project/gitlab
• Dropfort http://dropfort.com/
• Gittower

Mention that GitLab is compatible with any Git client by using git over ssh and/or git over http(s)