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
Homepage: omit youtube embed from short event list
Homepage: link event title to local detail page

This leaves just a small toggle button but feels more natural. Maybe
we should just make the button bigger?
Base template: add container for breadcrumbs

This makes it trivial to add breadcrumb nav by adding the additional
elements inside {% block breadcrumbs %}.

Breadcrumbs can be turned off entirely as on the homepage using an
empty block:

{% block breadcrumbs-container %}{% endblock breadcrumbs-container %}
Events: finish browser hierarchy

* Add breadcrumb nav throughout section
* Add date-based drilldown throughout section
* Add year and month-level listings