Event Fixups

Should close #74, but haven't verified that it does (see end). If we're editing the slug, we should verify the new slug is unique. Otherwise, we don't need to bother.

Also changed some error-checking logic in `app/models/Event.py`. I assumed we wanted `if self.start_date == self.end_date` and not `if self.start_date == self.start_time`. Is that correct?

Have not yet verified this works. Because I set GOOGLE_AUTH_ENABLED to false, I'm running into a lot of google calendar sync failures. Do you know what I should do to get rid of those failures? Should I write a TestGoogleApiClient that just simulates an actual Google Calendar?