Create prometheus mailq exporter + alert

Someone (cough) broke Postfix (!249 (merged)) on at least aur.archlinux.org, we should setup monitoring of the mail queue length.

The mail queue on aur.archlinux.org got in a vicious circle (2,7k mails in the queue) due to:

1. $6$rounds=656000 on the password on mail.archlinux.org causing every authentication to take 0,6 second
2. -> resulting in postfix authentication timing out (after 10 seconds when hitting mail.archlinux.org hard)
3. -> soft-bounce queued to notify@aur.archlinux.org (= more mails in the queue), which would result in another bounce as the mailbox didn't exists

We should monitor the queue.

mentioned in commit 9282c52d

mentioned in merge request !880 (closed)

mentioned in commit 96fdaf29

mentioned in commit 89d84e03