Keep Moderation History

Add an option to GenericModerator that controls whether history of moderated objects should be kept or not.

Add a new "date_updated" field to ModeratedObject so that we can determine what the latest object is. When keeping history there's no other way of uniquely finding the latest ModeratedObject. Migration file is also provided.

Update ModerationObjectManager.get_for_instance to try and get the moderated object the old way, and if there are many (because keep_history = True), find the most recent one and return that.

Added a test to make sure the post save handler works with keep_history
Added a test for ModeratedObjectManager to test get_for_instance