Extract Event::Search from EventsController#search

Classic extract-object refactoring here. Lots of yummy benefits:
* EventsController#search is now only concerned about controller stuff: flash, redirects, setting the view context, etc.
* EventsController#search is now only dealing with one object as its context, instead of many. Same with the View.
* New Event::Search object is now unit tested without involving a controller and the rest of the Rails stack.
* Event::Search is now a natural home to move some event-searching responsibilities off of Event in the future.