Events: local sync

Model refactor:
* Ensure that meetup IDs are unique
* Add auto_now / auto_now_add timestamps to Events
* Change meetup_id columns to text to match API responses
* Add meetup_event_sync management command
* Update field blank status based on existing data
Events: enable date-based views

* update Event model to switch to a slug field
* Add event detail view
* Change event list view to display simple tables for past and
  upcoming events
* Renamed templates to following generic view defaults
Events: work around time-zone interactions with URLs

The archive URLs are date-coded but because they're based off of a
timezone-aware DateTimeField there was the possibility of a 404 any time
the start_time was close enough the date boundary of the user's
timezone (see https://code.djangoproject.com/ticket/18794)

Since we want to use our calendar dates for the archive, this change
simply overrides the check to use only the year/month/day components of
the date.
Change homepage to use database events