Merge branch 'feature/toggle-pregrading' into demo

client/views/Exam.jsx

client/views/ExamEditor.jsx

client/views/Graders.jsx

migrations/versions/02f99246020f_grading_policy.py

migrations/versions/d33733e11085_grader_name_unique.py

tests/api/test_exams.py

tests/api/test_feedback.py

tests/api/test_grader.py

tests/api/test_mc_option.py

tests/api/test_widgets.py

tests/test_database.py

tests/test_scans.py

zesje/api/__init__.py

zesje/api/exams.py

zesje/api/graders.py

zesje/api/problems.py

zesje/blanks.py

zesje/database.py

zesje/image_extraction.py

zesje/pregrader.py