Add a link to the hub control panel

[yuvipanda]

Currently, there's no way to go back to the hub home page / control panel without modifying the URL. We should instead have a button in the UI that lets people go back home / to JupyterLab launcher.

[romainrossi]

I agree, the current behavior send the user to a new tab with no way to easily go back to the "main" jupyterlab UI.

I will be interested into integration of the VNC viewer in a tab, like the original NoVNC jupyterlab extension : https://github.com/robostack/jupyterlab-novnc

I tried using the original NoVNC extension to connect to the VNC server but with no success.

[yuvipanda]

@romainrossi with #78, you can click the jupyter logo to go back to lab / notebook!

I think it would still be nice to have a separate link to the hub control panel itself.