mattdrayer/api-course-avg-total-enrolled: Include ungraded students

Updated the course average calculation to account for enrolled students who have not yet registered a score, and therefore do not yet have entries in the gradebook.  We assume zero for their grades in this case and perform a simple mathematical adjustment to the average.  I also updated the main courses/metrics/grades operation to use the same model method to ensure consistent responses across both calls.

@chrisndodge, @martynjames, @ziafazal , FYI