Detach the downtime page from rails
From: https://gitlab.com/gitlab-com/infrastructure/issues/2266
We have noticed that when the application is in a state that cannot be started (missing NFS mount) we can't actually bring the deployment page up.
We should use a different mechanism to do this, probably doing it at the HAProxy level instead of relying on the monolith as when it can't start, we just can't do it.
We should use the opportunity to change the page to a downtime page instead of a deployment one and return a different error code to account for the unplanned downtime correctly.