Break apart ZoomTransitionController (2/2).

Update DayViewController. Remove the original
ZoomTransitionController aspect. Rename file to
ZoomTransition.swift.

Benefits: 

- No more weird additional referencing like in
ZoomTransitionController.

- No more close mingling of layout and transition
concerns.
Harden EventViewController error generation.

Forced unwrapping is mostly evil.

Also, better trimming.