Replace create_xmodule with create_xblock

@cpennington @nasthagiri please review

* The primary task of this is to do all of the import/export; so, the title's wrong
* reconciles the signature of create_xblock
* uses make_course_key from cale's branch
* uses nimisha's asset fixes
* lots of import/export fixes
`partitioned_fields` looks like `{Scope.settings: {'field_name': field_object}}`. We want just `{'field_name': field_object}`.