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/d33733e11085_grader_name_unique.py

+""" Ensures each grader is uniquely identified by name
+
+Revision ID: d33733e11085
+Revises: 02f99246020f
+
+"""
+from alembic import op
+from flask import current_app
+import shutil
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision = 'd33733e11085'
+down_revision = '02f99246020f'
+branch_labels = None
+depends_on = None
+
+
+def backup_db():
+    """
+    Creates a backup of the current database by making a copy
+    of the SQLite file.
+    """
+    db_url = current_app.config.get('SQLALCHEMY_DATABASE_URI')
+    db_path = db_url.replace('sqlite:///', '')
+
+    shutil.copy2(db_path, db_path + '.old')
+
+
+def upgrade():
+    backup_db()
+
+    conn = op.get_bind()
+
+    graders = conn.execute('SELECT id, name FROM grader').fetchall()
+
+    for grader in graders:
+        # Check if grader is not deleted already
+        if not conn.execute(f'SELECT * FROM grader WHERE grader.id  = {grader.id}').fetchall():
+            continue
+
+        # Get other graders with same name
+        other_graders = list(filter(lambda x: x[1] == grader[1] and x != grader, graders))
+
+        for other_grader in other_graders:
+            conn.execute(f'UPDATE solution SET grader_id = {grader.id} WHERE solution.grader_id = {other_grader.id}')
+            conn.execute(f'DELETE FROM grader WHERE grader.id = {other_grader.id}')
+
+    # Create copy table and remove data
+    op.create_table(
+        'grader_copy',
+        sa.Column('id', sa.Integer(), nullable=False),
+        sa.Column('name', sa.Text(), nullable=False, unique=True),
+        sa.PrimaryKeyConstraint('id')
+    )
+
+    op.execute('INSERT INTO grader_copy (id, name)' +
+               'SELECT id, name FROM grader')
+
+    op.drop_table('grader')
+    op.rename_table('grader_copy', 'grader')
+
+
+def downgrade():
+    backup_db()
+
+    # Add Grader table without unique constraint
+    op.create_table(
+        'grader_copy',
+        sa.Column('id', sa.Integer(), nullable=False),
+        sa.Column('name', sa.Text(), nullable=False, unique=False),
+        sa.PrimaryKeyConstraint('id')
+    )
+
+    # Move data from old Grader table
+    op.execute('INSERT INTO grader_copy (id, name)' +
+               'SELECT id, name FROM grader')
+
+    op.drop_table('grader')
+    op.rename_table('grader_copy', 'grader')