In graded_id I use self.id for same purpose. I think we should use
self.id (```i4x://x2/x3/lti/da1c627f934d4d529bf8333dd4e78967```) instead of self.location.html_id (``i4x-x2-x3-lti-da1c627f934d4d529bf8333dd4e78967```) here. @nedbat please correct me if I'm wrong.

Also, this value should be different for different instances of edX platform. If same course with LTI module located in same location, will be hosted at different edX instances, self.id (and self.location.html_id) will be different. @nedbat please confirm this.