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

-""" empty message
+""" Adds column to problem to specify the grading policy
 
 Revision ID: 02f99246020f
 Revises: b46a2994605b
@@ -16,14 +16,12 @@ depends_on = None
 
 
 def upgrade():
-    # ### commands auto generated by Alembic - please adjust! ###
+    # Add grading policy column to Problem
     op.add_column('problem', sa.Column('value',
                                        sa.Enum('set_nothing', 'set_blank', 'set_blank_single', name='gradingpolicy'),
                                        default=1, nullable=True))
-    # ### end Alembic commands ###
 
 
 def downgrade():
-    # ### commands auto generated by Alembic - please adjust! ###
+    # Remove grading policy column from Problem
     op.drop_column('problem', 'value')
-    # ### end Alembic commands ###