* chore: add migration to change timestamps to UTC
* chore: fill in Meeting.time_zone where it is blank
Nearly all interim meetings on or before 2016-07-01 have blank
time_zone values. This migration fills these in with PST8PDT.
* chore: disallow blank Meeting.time_zone value
* refactor: no need to handle blank time_zone case in TZ migration
* refactor: remove now-unnecessary checks that meeting has time_zone
* chore: renumber migrations
* chore: update timestamp conversion migration
The django-celery-beat package introduces tables with timestamp
columns. These columns are stored in CELERY_TIMEZONE. Because we run with
this set to UTC, the migration ignores these columns.
* chore: fix pytz-related change in migration
* chore: be explicit that Meeting.vtimezone can return None
* refactor: remove unnecessary save()
