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.

tests/api/test_exams.py

@@ -11,7 +11,7 @@ def add_test_data(app):
         db.session.add(exam1)
         db.session.commit()
 
-        problem1 = Problem(id=1, name='Problem 1', exam_id=1, grading_policy=1)
+        problem1 = Problem(id=1, name='Problem 1', exam_id=1)
         db.session.add(problem1)
         db.session.commit()