Feature requests: Notifications of unsubmission for participations, and ungraded sessions

Students frequently complaint about
1. forgetting to submit flows (with all page submitted)
2. not knowing there were assignments
3. having to navigate to all start page of flows to see whether there were unsubmitted sessions (for human graded flows, ``My Grades`` view can't differentiate whether the flow was not started or not finished or not graded)

I think integrating [django-notificiations](https://github.com/django-notifications/django-notifications) might be of help.