* Convert ObjectType to a concrete child model of ContentType
* Add public flag to ObjectType
* Catch post_migrate signal to update ObjectTypes
* Reference ObjectType records instead of registry for feature support
* Automatically create ObjectTypes
* Introduce has_feature() utility function
* ObjectTypeManager should not inherit from ContentTypeManager
* Misc cleanup
* Don't populate ObjectTypes during migration
* Don't automatically create ObjectTypes when a ContentType is created
* Fix test
* Extend has_feature() to accept a model or OT/CT
* Misc cleanup
* Deprecate get_for_id() on ObjectTypeManager
* Rename contenttypes.py to object_types.py
* Add index to features ArrayField
* Keep FK & M2M fields pointing to ContentType
* Add get_for_models() to ObjectTypeManager
* Add tests for manager methods & utility functions
* Fix migrations for M2M relations to ObjectType
* model_is_public() should return False for non-core & non-plugin models
* Order ObjectType by app_label & model name
* Resolve migrations conflict
