Assign owner to current user on import.

Per story [61087772](https://www.pivotaltracker.com/story/show/61087772).  Does this by adding an `after_save()` method to the base row converter classes, and having all (current) derived classes call it first.  This then add an `ObjectOwner` to the session.

NOTE: All `after_save` method definitions in derived classes in diverged branches need to be updated with that superclass call!

Also removes a call that did not seem to have a purpose and was crashing the object owner creation process (`self.dump_self` in `class ObjectOwner`).