Refactor views to viewsets

This still needs tests for SubmissionTypeApiView and for the StudentListView and for the DELETE method on Tutors (which otherwise has to be removed again).

Doing this is actually super helpful in providing a fairly simple interface for object creation.

backend/core/tests/data_factories.py

@@ -7,6 +7,7 @@ from core.models import (ExamType, Feedback, Reviewer, Student, Submission,
 # These methods are meant to be used to provide data to insert into the test
 # database
 
+
 def make_user(username='user01',
               password='p',
               fullname='us er01',