Move organize method out of the events controller

Jon, Shell, Desi and I started looking for an example of a refactoring we could do and found this - we moved a non-restful method out of the events_controller and into its own controller.
I think these tests are pretty weak sauce in this form; there's no behavior here, it's just stating that whatever a `volunteer_rsvps` is, we get it from Event and it gets sent to the view. I don't really see the value of locking that down, and it seems like if we cared about checking all the `assigns` we could do all three in one test rather than one-by-one. 

If nothing in the test is going to use the result of `volunteer_rsvps` in any realistic way it doesn't even need to be a real thing from the database, it could be a string or a spy or a bird or a plane.