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 wonder if there's a better name for this method, since it's not just based on branch_setting but also based on whether the category of the location is direct_only.  How about `set_branch`?