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/pregrader.py

@@ -89,13 +89,13 @@ def grade_as_blank(sol):
     sol : Solution
         The solution to pre-grade
     """
-    feedback = FeedbackOption.query.filter(FeedbackOption.problem_id == sol.problem.id,
-                                           FeedbackOption.text == 'blank').one_or_none()
-
     if sol.problem.grading_policy.value > 0:
         set_auto_grader(sol)
 
-    if feedback is None:
+    feedback = FeedbackOption.query.filter(FeedbackOption.problem_id == sol.problem.id,
+                                           FeedbackOption.text == 'blank').one_or_none()
+
+    if not feedback:
         feedback = FeedbackOption(problem_id=sol.problem.id, text='blank', score=0)
         db.session.add(feedback)