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.

tests/api/test_grader.py

+import pytest
+
+
+from zesje.database import db, Grader
+
+
+@pytest.fixture
+def add_test_data(app):
+    with app.app_context():
+        grader1 = Grader(name='grader')
+        db.session.add(grader1)
+
+        db.session.commit()
+
+
+# Actual tests
+
+
+@pytest.mark.parametrize('grader_name, expected_status_code', [
+    ('grader', 409),
+    ('grader2', 200)],
+    ids=['Duplicate grader name', 'Unused grader name'])
+def test_add_grader(test_client, add_test_data, grader_name, expected_status_code):
+    body = {'name': grader_name}
+
+    result = test_client.post('/api/graders', data=body)
+    assert result.status_code == expected_status_code