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

@@ -26,7 +26,7 @@ def grade_problem(sub, page, page_img):
     """
     solutions_to_grade = [
         sol for sol in sub.solutions
-        if not sol.graded_at and sol.problem.widget.page == page
+        if (not sol.graded_by or sol.graded_by.name == 'Zesje') and sol.problem.widget.page == page
     ]
 
     for sol in solutions_to_grade: