Coalesce swipe-to-unpop support into AwfulNavigationController.

Might as well put it all in one place. As a bonus, by moving the one-line
implementation of "show/hide toolbar depending on the view controller's
toolbarItems" here too, AwfulAppDelegate no longer has a reason to be any
navigation controller's delegate.
Move interactivePopGestureRecognizer fixups to AwfulNavigationController.
Don't immediately follow the link when revealing a linkified image.