handling multiple stories for an event

[frnsys]

currently, most of the functionality relating an event and a story assume that an event only belongs to one story. the backend has support for events belonging to multiple stories, since this is a likely scenario, and the front end should also support this.

some places where this needs work:

• in an event's actions, "Watch" defaults to watching the first story in an event
• in an event's page, the "previously in this story" just shows the previous events for the first story in that event