Pass in base_url rather than app object

- Limits what we consider public API
- Still allows for redirects outside JupyterHub

jupyterhub/app.py

@@ -1269,7 +1269,8 @@ class JupyterHub(Application):
         2. request - A Tornado HTTPServerRequest object representing the
            current request.
         3. user - The currently authenticated user.
-        4. app - The JupyterHub object
+        4. base_url - The base_url of the current hub, to allow for relative
+                      redirects
 
         It should return the new URL to redirect to, or None to preserve
         current behavior.

jupyterhub/handlers/base.py

@@ -1490,7 +1490,7 @@ class UserRedirectHandler(BaseHandler):
         if self.app.user_redirect_hook:
             url = await maybe_future(
                 self.app.user_redirect_hook(
-                    path, self.request, self.current_user, self.app
+                    path, self.request, self.current_user, self.base_url
                 )
             )
         if url is None:

jupyterhub/tests/test_pages.py

@@ -406,11 +406,11 @@ async def test_user_redirect_hook(app, username):
     name = username
     cookies = await app.login_user(name)
 
-    async def dummy_redirect(path, request, user, passed_app):
+    async def dummy_redirect(path, request, user, base_url):
-        assert passed_app == app
+        assert base_url == app.base_url
         assert path == 'redirect-to-terminal'
         assert request.uri == ujoin(
-            app.hub.base_url, 'user-redirect', 'redirect-to-terminal'
+            base_url, 'hub', 'user-redirect', 'redirect-to-terminal'
         )
         url = ujoin(user.url, '/terminals/1')
         return url