Fix #858: Broadcast/handle rejected follows

api/funkwhale_api/federation/api_views.py

api/funkwhale_api/federation/routes.py

api/funkwhale_api/federation/serializers.py

api/tests/federation/test_api_views.py

api/tests/federation/test_routes.py

api/tests/federation/test_serializers.py

changes/changelog.d/858.enhancement