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.

migrations/versions/02f99246020f_grading_policy.py

@@ -28,15 +28,8 @@ def upgrade():
         sa.PrimaryKeyConstraint('id')
     )
 
-    conn = op.get_bind()
-
-    old_problems = conn.execute('SELECT id, name, exam_id FROM problem').fetchall()
-
-    grading_policy = "'set_blank'"
-    for id, name, exam_id in old_problems:
-        # Add quotes for SQLite
-        name = "'" + name + "'"
-        conn.execute(f'INSERT INTO problem_copy VALUES ({id}, {name}, {exam_id}, {grading_policy})')
+    op.execute('INSERT INTO problem_copy (id, name, exam_id, grading_policy)' +
+               'SELECT id, name, exam_id, \'set_blank\' FROM problem')
 
     op.drop_table('problem')
     op.rename_table('problem_copy', 'problem')