Can now have multiple system actors

We also handle webfinger/activity serialization properly

api/tests/federation/test_authentication.py

@@ -0,0 +1,39 @@
+from funkwhale_api.federation import authentication
+from funkwhale_api.federation import keys
+from funkwhale_api.federation import signing
+
+
+def test_authenticate(nodb_factories, mocker, api_request):
+    private, public = keys.get_key_pair()
+    actor_url = 'https://test.federation/actor'
+    mocker.patch(
+        'funkwhale_api.federation.actors.get_actor_data',
+        return_value={
+            'id': actor_url,
+            'outbox': 'https://test.com',
+            'inbox': 'https://test.com',
+            'publicKey': {
+                'publicKeyPem': public.decode('utf-8'),
+                'owner': actor_url,
+                'id': actor_url + '#main-key',
+            }
+        })
+    signed_request = nodb_factories['federation.SignedRequest'](
+        auth__key=private,
+        auth__key_id=actor_url + '#main-key'
+    )
+    prepared = signed_request.prepare()
+    django_request = api_request.get(
+        '/',
+        headers={
+            'Date': prepared.headers['date'],
+            'Signature': prepared.headers['signature'],
+        }
+    )
+    authenticator = authentication.SignatureAuthentication()
+    user, _ = authenticator.authenticate(django_request)
+    actor = django_request.actor
+
+    assert user.is_anonymous is True
+    assert actor.public_key == public.decode('utf-8')
+    assert actor.url == actor_url