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

@@ -98,7 +98,7 @@ def set_blank_feedback(problem, sol):
         set_auto_grader(sol)
 
     if feedback is None:
-        new_feedback_option = FeedbackOption(problem_id=problem.id, text='blank')
+        new_feedback_option = FeedbackOption(problem_id=problem.id, text='blank', score= 0)
         db.session.add(new_feedback_option)
         db.session.commit()
         feedback = FeedbackOption.query.filter(FeedbackOption.problem_id == problem.id,