Toggling pregrading and Identifying blank solutions

This merge request does two things:

• Automatically identify blank solutions (either multiple choice or not) and set a feedback option named 'blank' for each one.
• When creating an exam, there are three options to specify which questions should be graded automatically.
• Pre-grade a question again if a submission is reuploaded and is automatically graded (by 'Zesje')

The grading policy is specified in the following way:

• 0/set_nothing: Manually approve everything
• 1/set_blank: Automatically approve blank solutions
• 2/set_blank_single: Automatically approve blank solutions or solutions with one answer

Add a new field to a problem in the database with this number.

For a more detailed description, see #61.

Closes #33 and #61.

zesje/api/problems.py

@@ -98,20 +98,19 @@ class Problems(Resource):
             the attribute (or property) to put to
 
         Returns
-            HTTP 200 on succes, 404 if problem is invalid
+            HTTP 200 on success, 404 if problem is invalid
         """
 
         args = self.put_parser.parse_args()
 
         problem = Problem.query.get(problem_id)
+
         if problem is None:
-            msg = f"Problem with id {problem_id} doesn't exist"
-            return dict(status=404, message=msg), 404
+            return dict(status=404, message=f"Problem with id {problem_id} doesn't exist"), 404
 
-        for key in args.keys():
-            value = args[key]
+        for attr, value in args.items():
             if value is not None:
-                setattr(problem, key, value)
+                setattr(problem, attr, value)
 
         db.session.commit()