Adding course progress information to the LMS dashboard

One of our customers reports that students frequently ask for an overview of their course progress being shown in the dashboard.  As a first step, we simply want to show the current grade percentage on that page.  Exactly how the UI would change is still to be worked out – either we just show the progress in percent, or we show some graphical progress bar.

The feature will be hidden behind a feature flag, so it wouldn't necessarily show up on edx.org.

To be able to implement this efficiently, we plan to retrieve the current grade by building a CourseGrade instance using

    CourseGradeFactory().read(user, course_key=course_key)

If the grading policy of the course changed, this progress information might be out of date, but according to some crude tests we did recomputing the progress for every course would be too slow for the dashboard.