Add pinch recognizer to NavigationController.

Only set up for controllers presenting collection
vc's.
Factor out ZoomTransition from ZoomTransitionCoordinator.
Rename ZoomTransitionCoordinator to ZoomTransitionController.

'Transition coordinator' is already an owned UIKit
term and method of UIViewController.

Not sure what I was smoking...
Remove unnecessary protocol implementations from ZoomTransitionController.

Again, not sure what I was smoking, trying to
implement my own transitionContext.
Remove unused ZoomTransitionController property.