Add a command to Shut Down the server (and open the jhub-apps?)

[krassowski]

Currently closing the server requires a couple of clicks. The "Shut Down" menu entry could be embedded in the "File" menu instead. Maybe it should be "stop server and open jhub-apps main page"

[krassowski]

Transferring this issue from jupyterhub-nebari-mode because it makes more sense to implement it here.