Enhancement/wrobe0709/list unenrolled submissions

[wrobe0709]

Fixes #1070

If a student is not enrolled in a class but submitting assignments for the class their submissions won't be seen by the staff.

This adds a view called "Un-Enrolled Submitters" which allows staff members to see any students who are submitting assignments to the course but are not enrolled.

[wrobe0709]

I'm pulling in staff and lab assistants because I was getting them in the list of un-enrolled submitters. I'm assuming we don't want them in that list and that we just want students?

Because I can get the user objects from get_students(), get_staff(), and get_participants([LAB_ASSISTANT_ROLE])) I made submissions a set of user objects. Does that look like the best way to do it?