Increase Event::Saver cohesion

My favorite kind of refactoring: almost all red!

I noticed that `Event#associate_with_venue` and `Venue.find_by_identifier` were only being consumed by `Event::Saver`, so I moved them there, and made them private. Once that was done, it was easy to refactor the whole shebang down to one small method. Textbook cohesion!

The other benefit is that we were previously testing these effectively private methods, so now we can kill all those tests and still retain 100% test coverage of all three classes.