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

Auto-migration and everything seems to work. It's just that
matrix-registration cannot load the Python modules required
for talking to a Postgres database.

Tracked here: https://github.com/ZerataX/matrix-registration/issues/44

Until this gets fixed, we'll continue default to 'sqlite'.