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
I believe Studio's new Publishing UI calls `compute_publish_state` only for the purpose of determining whether a module is also Published.  @benmcmorran Can verify this.  That is, if not all 3 states are used by Studio (Public, Private, Draft), we can change the semantics of the method.