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

@@ -90,7 +90,7 @@ class Problems(Resource):
     def put(self, problem_id):
         """PUT to a problem
 
-        This method accepts any of the legible arguments passed to it.
+        This method accepts both the problem name and the grading policy.
 
         problem_id: int
             the problem id to put to
@@ -98,7 +98,7 @@ class Problems(Resource):
             the attribute (or property) to put to
 
         Returns
-            HTTP 200 on success, 404 if problem is invalid
+            HTTP 200 on success, 404 if the problem does not exist
         """
 
         args = self.put_parser.parse_args()