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.
Remove Xcode beta workaround.
Remove custom CSS feature.
Actually remove effect of "Show Thread Tags" setting.

I knew the cells wouldn't do anything, but I forgot about the table view code
that wouldn't set the image.
Remove interface theming.

Gonna redo it, but for now, it's just clutter.
Set app-wide tint color to something visible.
Hide separators in Leper's Colony table.
Paint the iPad an Awful blue.
Refresh forums on appear when network becomes reachable.

This behavior was previously inherited from AwfulFetchedTableViewController,
but the forums list no longer uses that.