Fix split view autoresizing masks.

Sidebar height wasn't being set properly when launching in landscape.
Formatting and simplifying.
Clarify -[AwfulSplitViewController setSidebarCanHide:]
Remove redundant layout in split view controller.
Fix tab bar covering bottom of selected view.

The fix was basically to re-layout the content when the view appears, and after
rotation. I simplified some of the code as I was debugging.
Draw navigation bar background in code.
Move navigation bar appearance customization out of app delegate.
View controllers set their own modal presentation style.
Fix and simplify two-line title in navigation bar.

Since moving the compose reply button up to the navigation bar, there's always
a left item and right item whenever the two-line title is shown. So it makes
sense to center the text again, and that means we can just let UINavigationBar
do its thing. Happily, that fixes reported issues with title layout when
rotating an iPhone while a YouTube video is fullscreen.