Add completion criterion of "student achieves passing grade in course"

Currently we have a completion criterion: "student achieves final grade of x." That final grade seems to expect a "sum of grades" calculation, though, which may no longer be accurate, since sum of grades was removed as an aggregation method. If we could have a completion criterion of "student achieves passing score (as defined in gradebook) by Course End date" and generate the completion event (or not) on that date, we could use it in learning analytics models as well as other situations that rely on course completion.