[Courses] Autodelete 'Test Student' from courses after set delay

Problem statement:

The dummy user 'Test Student' is created in courses when Student View is used. This user is then shown unnecessarily in SpeedGrader and the gradebook. Teaching staff who don't want this shown in these locations either have to filter the user out by selecting a section in SpeedGrader and the gradebook, or by searching Settings > Sections and manually deleting the user. They then need to repeat this if a staff member selects Student View again.

Proposed solution:

Remove this dummy user after a set delay, eg. 1 day. Optionally also allow clients to specify the delay after which the user should be deleted.

User role(s):

instructor,ta