As a guest, I can search for shows by title

* Create search form partial and render it in the application layout
* Add route for search
* Create `SearchResultsController`
* Add `search` class method to `Event` to search by event title
* Create `search_results/show` view

https://trello.com/c/dlYhCHRf
Add migration and model for event preferences

* Validate presence of `event` and `user` for `EventPreference`
* Add relationships to `Event` and `User`
Validate uniqueness of user within socpe of event
Implement ability for users specify event prefs

* Add route and controller for event preferences
* 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