During the last update, `hydra-notify` was rewritten as a daemon which
listens to postgresql notifications for each build[1]. The module
uses the `hydra-notify.service` unit from upstream's Hydra module and
the VM test ensures that email notifications are sent properly.
Also updated `hydra-init.service` to install `pg_trgm` on a local
database if needed[2].
[1] c7861b85c4
[2] 8a0a5ec3a3
ce37a040c2