Feat: Support for grading events

[bmtcril]

We would like to transform and capture some more grading events to help with downstream reporting:

• edx.grades.subsection.grade_calculated
• edx.grades.course.grade_calculated

These should be similar to the problem interaction events, but the objects would be a subsection or course instead of individual problems.

[Ian2012]

Another important grading event is: edx.course.grade.passed.first_time or edx.course.grade.now_passed for approvals

[bmtcril]

Yep, we should already have passed.first_time, now_passed, and now_failed.