store relationship between oauth client and service/spawner

so that we can look up the spawner/service from the oauth client and vice versa

jupyterhub/tests/test_api.py

@@ -307,7 +307,7 @@ async def test_get_self(app):
     db.commit()
     oauth_token = orm.APIToken(
         user=u.orm_user,
-        client=oauth_client,
+        oauth_client=oauth_client,
         token=token,
     )
     db.add(oauth_token)