Closes #155

core/models/assignment.py

core/models/submission.py

core/serializers/feedback.py

core/signals.py

core/tests/test_custom_subscription_filter.py

core/tests/test_feedback.py

core/tests/test_subscription_assignment_service.py

core/tests/test_tutor_api_endpoints.py

core/urls.py

core/views/feedback.py

core/views/subscription.py

core/views/util.py

frontend/src/api.ts

frontend/src/store/modules/submission-notes.ts

functional_tests/test_export_modal.py