Add a message on 503 error page

[slinlee]

The message should tell users the server is still being set up and should be available soon.

[maxpumperla]

@slinlee I don't think this is something we can solve "in" policy server. I guess there needs to be another web server that listens for policy server and renders a page as long as it's not up. When it's finally up, it should redirect.

Now, it's in principle possible to somehow do this from Python, but given that we already have a canonical candidate (the PM web app), I think that should be handled there. What do you think, @slinlee ?

[slinlee]

Yeah agreed on the approach. I filed this here because it fits here as a 'product'

Either way it's a much lower priority now because the web app deployment UI checks and notifies the user when the endpoint is ready.