Can now have multiple system actors

api/funkwhale_api/federation/actors.py

api/funkwhale_api/federation/authentication.py

api/funkwhale_api/federation/keys.py

api/funkwhale_api/federation/serializers.py

api/funkwhale_api/federation/urls.py

api/funkwhale_api/federation/views.py

api/funkwhale_api/federation/webfinger.py

api/tests/federation/test_actors.py

api/tests/federation/test_authentication.py

api/tests/federation/test_keys.py

api/tests/federation/test_serializers.py

api/tests/federation/test_views.py

api/tests/federation/test_webfinger.py