Source flag to indicate single events vs. ongoing calendar feed

[dhedlund]

Need a flag associated with the Source model that indicates whether the source is associated with a single event (some Facebook URLs, meetup.com, etc) or whether it should be monitored on an ongoing basis, even if there are no future events being returned by the feed. We could count the number of events associated with the source (Source#events.count), but it it can only prove that it is a multi-event source, not that it isn't; a multi-event source could still return a single event.

How this flag will be used: Single event sources will stop being monitored as soon as the event is over and multi-event sources will stop being monitored after a period of inactivity without any future events listed (3 months) and may be monitored less frequently (once every 3-5 days) if there are no upcoming events. Monitoring and expiring of sources is out of scope for this ticket.