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/tasks/validate_config.yml

@@ -18,10 +18,3 @@
   when: "item.old in vars"
   with_items:
     - {'old': 'matrix_registration_docker_repo', 'new': 'matrix_registration_container_image_self_build_repo'}
-
-- name: Fail if Postgres usage attempted
-  fail:
-    msg: >
-      matrix-registration doesn't support using Postgres just yet.
-      Learn more here: https://github.com/ZerataX/matrix-registration/issues/44
-  when: "matrix_registration_database_engine == 'postgres'"