Gitlab can show our alertmanager alerts only for > reporter and create
issues from alerts on gitlab.

We want to get notifications of pacman/arch-audit notifications grouped
as otherwise we'll be spammed with ~ X emails for every host.

Closes: #191

Record the rebuilderd queue length in prometheus so we can generate an
alert for when the queue length keeps rising. As this could be an
indication that the rebuilders have builds which are stuck.

Introduce a new monitoring server with prometheus and alertmanager for
monitoring all our boxes.