Private message table view cell.

Reusing the thread cell kind of made sense once upon a time, but they really
should be treated as two different things.
Forget displayed favorite forums on log out.
Only show Private Messages tab when user has Platinum.

Before, we left the tab in regardless because three tabs looks stupid. Now,
having switched to basement/vertical tabs, it looks less stupid.
Hide "Show Thread Tags" setting.

As of iOS 7-ification it has no effect.