Mirror underlay repositories in Gitolite so pushes to GitLab will trigger website refreshes

For now, we want to maintain the Gitolite hook machinery that triggers website refreshes. In order to to that, GitLab has to mirror all repositories that trigger website refreshes in Gitolite.

A refresh of the Tails website is triggered by:

• pushing to the master branch of our Git repository -- #57 (closed)
• pushing to a few "underlay" repositories.

This issue is about mirroring the underlay repositories.