JupyterLab is provided side-by-side with Jupyter Classic interface with same process.

Published URL changed:

 * jupyterlab-url provides IPv6 URL to JupterLab interface
 * jupyter-class-url provides IPv6 URL to the classic UI

Thanks to using only one process, the path in the URL differentiates the style of the interface.

Note: This MR replaces closed !207

/reviewed-on !218