Reorganize Postgres access (breaking change)

In short, this makes Synapse a 2nd class citizen,
preparing for a future where it's just one-of-many homeserver software
options.

We also no longer have a default Postgres superuser password,
which improves security.

The changelog explains more as to why this was done
and how to proceed from here.

docs/configuring-playbook-external-postgres.md

@@ -4,6 +4,7 @@ By default, this playbook would set up a PostgreSQL database server on your mach
 If that's alright, you can skip this.
 
 If you'd like to use an external PostgreSQL server that you manage, you can edit your configuration file  (`inventory/host_vars/matrix.<your-domain>/vars.yml`).
+
 It should be something like this:
 
 ```yaml