expand on `grade_notification_cache`

[elijaholmos]

currently implementation is only utilized during user creation, to store grades that have already been published prior to account connection.

expand implementation to prevent the duplication of grade notifications. do consider, however, that professors can remove & reupload grades for the same assessment. I am not sure if UserCourseClassAssessmentGrade objects have their own unique ID, and whether or not this ID changes when grades are removed & reuploaded.

[elijaholmos]

I believe another use case could be additional notification security:

• When a new grade is, for some reason, not detected in the HaloWatcher interval, this safety net would catch it and still provide a notification
• If the grade status is PUBLISHED and the grade ID does not appear in the grade_notification_cache, then we know the user has not been notified, and should we be clear to send a notification
  • what about users with notifications disabled via settings?