Fix ClusterableModel.__init__ to accept kwargs for relations that are defined on the model's superclass
Ensure that relations from superclasses are also saved in ClusterableModel.save
Ensure that relations from superclasses are serialized by ClusterableModel.to_json
Ensure that relations from superclasses are deserialized by ClusterableModel.from_json
cache the result of get_all_child_relations, since we use it all over the place
update changelog entry, since it wasn't just ClusterForm that got child_relations wrong