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

-""" Adds column to problem to specify the grading policy
-
-Revision ID: 02f99246020f
-Revises: b46a2994605b
-
-"""
-from alembic import op
-import sqlalchemy as sa
-
-
-# revision identifiers, used by Alembic.
-revision = '02f99246020f'
-down_revision = 'b46a2994605b'
-branch_labels = None
-depends_on = None
-
-
-def upgrade():
-    # Create copy of problem table
-    op.create_table(
-        'problem_copy',
-        sa.Column('id', sa.Integer(), autoincrement=True, nullable=False),
-        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'),
-                  server_default='set_blank', default='set_blank', nullable=False),
-        sa.ForeignKeyConstraint(['exam_id'], ['exam.id'], ),
-        sa.PrimaryKeyConstraint('id')
-    )
-
-    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')
-
-
-def downgrade():
-    # Create copy of old problem table (without grading policy)
-    op.create_table(
-        'problem_copy',
-        sa.Column('id', sa.Integer(), autoincrement=True, nullable=False),
-        sa.Column('name', sa.Text(), nullable=False),
-        sa.Column('exam_id', sa.Integer(), nullable=False),
-        sa.ForeignKeyConstraint(['exam_id'], ['exam.id'], ),
-        sa.PrimaryKeyConstraint('id')
-    )
-
-    # Move data from new problem table into old problem table
-    op.execute('INSERT INTO problem_copy (id, name, exam_id)' +
-               'SELECT id, name, exam_id FROM problem')
-
-    op.drop_table('problem')
-    op.rename_table('problem_copy', 'problem')