as the last remaining objective-c file, this removed direct dependency
from the SignalMessaging framework headers on SignalServiceKit and
thus required adding lots of import statements
Also convert OWSTableContents, OWSTableSection and OWSTableItem to Swift.
The only subclass of OWSTableViewController was DebugUITableViewController which has to be converted to Swift as well.
I also changed OWSTableViewController to be a subclass of OWSViewController (more modern and all in Swift) instead of OWSViewControllerObjc.
