title: AppEngine Datastore Admin does not work with federated login
body: <p>When I click on "Datastore Admin" in my AppEngine (Python27) application with federated login, the following error shows up:</p>
  
  <pre><code>Error: Not Found
  
  The requested URL /_ah/login_required?continue=https://ah-builtin-python-bundle-dot-latest-dot-ah/datastore_admin/?app_id=s~lpgng2 was not found on this server.
  </code></pre>
  
  <p>Seems like I am redirected to the /_ah/login_required page on admin instance and not on the frontend instance.</p>
  
  <p>What's wrong?</p>