Keep History - test post save handler

Add a new "date_updated" field to ModeratedObject so that we can determine what the latest object is.
Update ModerationObjectManager.get_for_instance to try and get the moderated object the old way,.and if there are too many (because keep_history = True), find the most recent one and return that.
Add a test to make sure the post save handler works with keep_history
Add a test for ModeratedObjectManager to test get_for_instance