Add get_absolute_url to Event

Idiomatic Django:
https://docs.djangoproject.com/en/1.7/ref/models/instances/#get-absolute-url
Remove destructive GET RSVP URL

After some discussion, and some research into how other sites deal with
carrying intentions through a login cycle I decided to remove the
special case, destructive GET request for RSVPing to events. This
simplifies the code and makes RSVPing to event work exactly like
following groups.
Merge pull request #210 from azavea/fix/jcw/remove-rsvp-destructive-get

Remove destructive GET request from RSVP workflow