Break apart ZoomTransitionController (1/2).

Better separate transition concerns per existing
UIKit protocols. Update MonthsViewController.

- Create TransitioningDelegate and
AnimatedZoomTransition. The former creates the
latter.

- Refactor zoomedOutView into a
snapshotReferenceView delegate method.

- Add TransitionAnimationDelegate to replace
ZoomTransitionControllerDelegate.
Fixup 0a9ad69: remove nonworking, accidentally committed code.