Cleanup of Group usage

gracedb/alerts/tests/test_views.py

gracedb/api/v1/events/views.py

gracedb/api/v1/superevents/serializers.py

gracedb/api/v1/superevents/views.py

gracedb/events/models.py

gracedb/events/view_logic.py

gracedb/events/views.py

gracedb/ligoauth/middleware.py

gracedb/ligoauth/tests/test_backends.py

gracedb/ligoauth/tests/test_middleware.py

gracedb/search/tests/test_access.py

gracedb/superevents/mixins.py

gracedb/superevents/utils.py