Make the config initialization script run in gitlab.service's PreStart
section into two new services, `gitlab-config.service` and
`gitlab-db-config.service`. Other services can then depend on the
config scripts they need instead of unnecessarily depending on
`gitlab.service`. This makes the reason for the configured service
dependencies much clearer and should also reduce the restart time of
the `gitlab` service quite a lot, when triggered manually.
Also, set up stricter service dependencies, using `bindsTo`, to ensure
that if a service fails or is stopped, its dependants are also
stopped. For example, if we're using the `postgresql` service and it's
stopped, `gitlab.service` and `gitlab-sidekiq.service`, which depend on
it to function, should also be stopped.
