Add a simple /tree handler.

[afshin]

This PR provides a handler for all URLs that begin with the tree_url prefix. The default value for tree_url is /lab/tree/. The handler returns exactly what the LabHandler returns for HTTP get requests because it is simply an instance of the LabHandler that matches a permissive regular expression.

The JupyterLab front-end will need to read the URL and decide how to manifest the URL in the application state (e.g., open a folder or a notebook, etc.): jupyterlab/jupyterlab#3396

[ellisonbg]

I posted this comment on jupyterlab/jupyterlab#3396 as well:

We should add a low priority handler to /lab/* that makes sure that any 404 with the /lab prefix routes the user to lab, and not the default server 404 page.

[ellisonbg]

Add low-priority /lab/* handler that redirects to /lab

[blink1073]

Thanks!