Add the idea of an event

[dirn]

In order to reuse the site every year without clearing out Application's table
every time we need the concept of an event.

Event should be a straightforward model to implement. It will need a name (and
probably a slug, although how that will be utilized is best left to another
issue), a date range for accepting applications, and possibly a cutoff date for
early bird applications (I'm not sure if this will be used for anything
functional, but it's a piece of data we've displayed in the past). We'll
probably need additional fields in the future (e.g., an identifier from the
registration service to create discount codes and/or tickets), but we'll worry
about that when we know who and what that will be.

Event should be registered as a foreign key on Application. In order to ease
the migration, a default event should be created and associated with any
existing applications.

Another nice-to-have would be a way to get the "current" Event when one hasn't
been explicitly specified. Initially maybe this can just be the one created by
the migration; down the road it can look at the dates.