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
To have the code be clear, I suggest not passing `course_key` to the `get_branch_setting` method since it's not used and makes one think the return value is related to the input.