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

@@ -23,7 +23,7 @@ def upgrade():
         sa.Column('name', sa.Text(), nullable=False),
         sa.Column('exam_id', sa.Integer(), nullable=False),
         sa.Column('grading_policy', sa.Enum('set_nothing', 'set_blank', 'set_blank_single', name='gradingpolicy'),
-                  default=1, nullable=False),
+                  server_default='set_blank', default='set_blank', nullable=False),
         sa.ForeignKeyConstraint(['exam_id'], ['exam.id'], ),
         sa.PrimaryKeyConstraint('id')
     )