Previously, the course grade returned would be the stored grade,
which is calculated for all content, not just the visible grades.
(Some grades are not yet released to the learner.)
This fix recalculates the overall grade before sending to the MFE,
so that it doesn't have to recompute it itself (without all the
particular logic that the platform uses when grading).
AA-1217
* Add unsubscribe_token uuid field to CourseGoal model
* Add endpoint to unsubcribe from just a token (no login needed)
* Add admin page for the course_goals djangoapp
* Add get_course_overview_or_404 utility method
* Clean up URL handling in course_home_api
AA-907
