Undo Jupyter ServerApp usage to fix jupyter-nbcontrib-extensions

jupyter-nbcontrib-extensions required the following fixes to
work:

1. Pin nbclassic<0.5 (done in
   2i2c-org/utoronto-image#49, until
   Jupyter-contrib/jupyter_nbextensions_configurator#152
   is merged and released)
2. Upgrade jupyter-nbcontrib-extensions to latest released version
   (done in 2i2c-org/utoronto-image#48)
3. Stop using ServerApp, and use NotebookApp (issue to be filed here)

Ref 2i2c-org/utoronto-image#48

config/clusters/utoronto/default-common.values.yaml

@@ -1,8 +1,12 @@
 jupyterhub:
   singleuser:
+    extraEnv:
+      # Required to get jupyter-contrib-nbextensions to work
+      # See https://github.com/2i2c-org/infrastructure/issues/2380
+      JUPYTERHUB_SINGLEUSER_APP: "notebook.notebookapp.NotebookApp"
     image:
       name: quay.io/2i2c/utoronto-image
-      tag: "dfd2313d2617"
+      tag: "c68f2eb785f9"
   hub:
     config:
       Authenticator: