Add Quit button to exit JupyterLab

[fcollonval]

For the classical Notebook, a Quit button has been introduced to shutdown the Notebook server
(see jupyter/notebook#3004).

I though this is a good addition to make Jupyterlab behave more closely to a desktop app. But for other use cases (like online installation) this is probably a bad thing - I presume this is the reason for the newly introduced parameter NotebookApp.shutdown_button. But as my web-development skills are small, I for now has implemented it as an extension https://github.com/fcollonval/jupyterlab_quit. But for consistency with the classical notebook, it's a nice to have.

Nota: I may try to set a PR if somebody can point me how I could condition UI elements on config parameters.

[fcollonval]

Done in #5226. To be available in 0.35