[API] Expose course level statistics via API

[patdura]

There is some valuable information about course statistics available via the UI including the running totals which show the total number of active students and number of unaccepted students.  https://canvasdomain/courses/:courseid/statistics

It would be valuable to add an additional statistic which is # of students who have completed all of the module requirements.

But more importantly, it would be useful to have an API endpoint so that we can retrieve these statistics programmatically.