changed model so that exam_id cannot be null

core/migrations/0012_auto_20190306_1611.py

core/models.py

core/tests/test_access_rights.py

core/tests/test_commands.py

core/tests/test_factory.py

core/tests/test_feedback.py

core/tests/test_functional_views.py

core/tests/test_student_page.py

core/tests/test_subscription_assignment_service.py

core/tests/test_tutor_api_endpoints.py

core/tests/test_user_account_views.py

util/factories.py

util/importer.py