Tweak user_redirect_hook API

- Pass in user object & request object only explicitly.
  Much better interface that is harder to break by internal
  refactoring. We can always add more  parameters if needed?

jupyterhub/handlers/base.py

@@ -1488,7 +1488,7 @@ class UserRedirectHandler(BaseHandler):
         # processing
         url = None
         if self.app.user_redirect_hook:
-            url = await self.app.user_redirect_hook(self, path)
+            url = await maybe_future(self.app.user_redirect_hook(self.request, self.current_user))
         if url is None:
             user = self.current_user
             user_url = url_path_join(user.url, path)