Weblate: pull/push from/to GitLab (#55, #56)

Tails is now using GitLab to host its canonical repositories, and
Weblate has to use those repos to pull from and push to.

When pulling, we just change the origin remote URL to GitLab's.

When pushing, before this change, an update hook in Gitolite's tails.git
took care of preventing Weblate from modifying things it was not allowed
to. As we don't have SSH access to the root filesystem of the GitLab VM,
there's no simple way to setup Server Hooks there. Because of this, we
use a "gatekeeper" repository to do the same checks as before and proxy
updates to GitLab. As the gatekeeper is hosted in a different VM than
Weblate, we are still protected from privilege escalation as we were
before.