Add support for submission comment editing/deleting

[laitingsheng]

What resource needs additional coverage? Submission Comment

What endpoints need to be covered? PUT /api/v1/courses/:course_id/assignments/:assignment_id/submissions/:user_id/comments/:id DELETE /api/v1/courses/:course_id/assignments/:assignment_id/submissions/:user_id/comments/:id They are documented here.

After searching the repository, I realised that this endpoint (and two available methods) was not covered.