Hard-code settings.TIME_ZONE to UTC

[jnm]

We always want to deal with UTC, and we want the UTC offset (+00) to be specified explicitly in the database.

With TIME_ZONE = 'America/New_York, it's easy to mistakenly apply a time zone offset to something that was already UTC. Example:

>>> u.date_joined = datetime.datetime.now()
>>> u.date_joined  # this is already UTC!
datetime.datetime(2021, 3, 10, 22, 42, 55, 843948)
>>> u.save()
>>> u.refresh_from_db()
>>> u.date_joined  # Django assumed it was `America/New_York` and added 5 hours!
datetime.datetime(2021, 3, 11, 3, 42, 55, 843948, tzinfo=<UTC>)

With TIME_ZONE = 'UTC', Django behaves the way we want:

>>> u.date_joined = datetime.datetime.now()
>>> u.date_joined
datetime.datetime(2021, 3, 10, 22, 44, 0, 673962)
>>> u.save()
>>> u.refresh_from_db()
>>> u.date_joined
datetime.datetime(2021, 3, 10, 22, 44, 0, 673962, tzinfo=<UTC>)

Originally posted by @jnm in #677 (comment)