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.
Fix swipe-from-left-edge after cancelling swipe-from-right-edge.

This was already fixed but I screwed it up when moving it to AwfulNavigationController.
Formatting.
Fix the avatar atop the basement not updating properly.

NSURLSessionDownloadTask won't overwrite existing files. Which makes sense, I just didn't think of it.
Animate avatars atop basement.
Formatting.
Handle UIInterfaceOrientationUnknown in AwfulSplitViewController.

New in iOS 8, picked up by compile error.