MNT: Add fields for hub config info in JSON blob.

jupyterlab/lab.html

@@ -17,6 +17,8 @@ Distributed under the terms of the Modified BSD License.
 
   <script id='jupyter-config-data' type="application/json">{
   "baseUrl": "{{base_url | urlencode}}",
+  "hubHost": "{{hub_host | urlencode}}",
+  "hubPrefix": "{{hub_prefix | urlencode}}",
   "wsUrl": "{{ws_url| urlencode}}",
   "notebookPath": "{{notebook_path | urlencode}}"
   }</script>