Make matrix-registration use Postgres by default

Now that 0.7.2 is out, the Docker image supports Postgres
and we can do the (SQLite -> Postgres) migration.

I've also found out that we needed to fix up the `tokens.ex_date` column
data type a bit to prevent matrix-registration from raising exceptions
when comparing `datetime.now()` with `ex_date` coming from the database.

Example:

> File "/usr/local/lib/python3.8/site-packages/matrix_registration/tokens.py", line 58, in valid
> expired = self.ex_date < datetime.now()
> TypeError: can't compare offset-naive and offset-aware datetimes

roles/matrix-registration/defaults/main.yml

@@ -88,7 +88,6 @@ matrix_registration_admin_secret: ""
 
 matrix_registration_riot_instance: "https://riot.im/app/"
 
-
 # Default matrix-registration configuration template which covers the generic use case.
 # You can customize it by controlling the various variables inside it.
 #