LEARNER-76

Add a new waffle switch that allows us to assume zero grades for
learners who have no entry previously recorded, and another to
disable persisting grades for unengaged learners.

TNL-6691

Also adds JwtAuthentication to the list of allowed authentication
methods, so the Grades REST API can be easily accessed from Enterprise
management commands.

Remove deprecated SingleSectionGrader.  TNL-5987
Remove display_name and module_id from Scores objects
Update CourseGradeFactory.__init__ to not be user-specific
Update some callers to use CourseGrade class instead of "summary" dict
Remove no longer needed course_grades.py module.
Renamed django signal from GRADES_UPDATED to COURSE_GRADE_CHANGED

Initial commit

TNL-5484

This copies the grading policy endpoint currently in the
course_structures django app to its new location, in preparation
to remove the old app.

TNL-5701