As a user, I can indicate whether I want to see a show.

* Add migration, model, route, and controller for event preferences
* Add validations to `EventPreference`
* Add relationships to `Event` and `User`
* Add helper method to `User` to create or update an event preference
* Create partials for displaying the preference links on `events#show`
* Add predicate methods to `User` for each of the cases:
  * User has not indicated a preference for an event
  * User has indicated a positive preference for an event
  * User has indicated a negative preference for an event

https://trello.com/c/M24oab13
Probably could wrap this up into a reusable `preference_for(event)` method.