Disallow changing of the Gitlab username

See merge request archlinux/infrastructure!328

As keycloak is the canonical source of truth and interacting with gitlab
usually requires the username as identifier. As we don't want to
accidently assign permissions to a changed username disallow changing of
usernames altogether in Gitlab.

For symmetry with synapse-worker@.service.

To prevent mjolnir from failing.

Borg backup check

See merge request archlinux/infrastructure!322

Now that our textcollector depends on the condition of /backup being
available, we should be extra careful that we get rid of the directory.
If anything in the script fails, a trap now umounts and removes the
directory automatically if on btrfs.being available, we should be extra
careful that we get rid of the directory. If anything in the script
fails, a trap now umounts and removes the directory automatically if on
btrfs.

Currently our textcollector can sometimes fail with 'Failed to
create/acquire the lock /home/backup/$server/lock.exclusive (timeout)."
Instead of checking on a borg lock file, check if our backup snapshot
dir exists which the backup script creates and removes. This should give
less false positives then our current method.

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See merge request archlinux/infrastructure!319