Refactor events

Trying to make the code friendlier to future
developers.

*Clean up tests in event_spec
*Rename user methods in event so it is clear
 they are returning hashes
*Rewrite feature spec for RSVPing to an event
Remove event_organizers table

This table was really just a subset of event_rsvps and was
causing duplicate records if a user had an rsvp to an event
and then was made an organizer.

*Drop event_organizers table
*Add organizer boolean column to event_rsvp
*Eager load event_rsvps in events_controller#show
*Add methods to make a user an event organizer