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
Is the added `kwargs` on `update_item` still needed here?  (Although it doesn't matter since we added it in the "make split work with CMS" PR.