Add ability to have multiple 'base' paths in the file browser, and option to show their true paths - Mirror from JupyterLab

[chippey]

On advice from @krassowski on jupyterlab/jupyterlab#17782, mirror posting this over here to see if you're open to this proposal:

Problem
On a multi-user JupyterHub installation, there are a few shared storage locations users need to access for data and notebooks from their Jupyter Lab logins. The current file browser makes is impossible to easily browse and open files from multiple places (mounts, network locations, etc). Plugins like jupyter-fs are not viable as they appear to be incompatible with the way JupyerHub works. Setting the notebook_dir to the root of the filesystem is undesirable, as most of the locations are uninteresting to the user and just clutters up the listing (like /etc, /var, /usr etc etc ). This also creates more click-thru to get to desired places. Sym-linking the locations into the users' home folders is also undesirable as then their home folder becomes a prefix in the path, which causes problems for other users who might want to use someone else's setups from a shared location.

Proposed Solution
Allow multiple assigned notebook_dir's to be set, either as a list, or multiple times. Also allow the actual path to be shown as an option, so it's easy to make clear where each location is, instead of multiple pseudo root / paths.

Additional context
A quick mockup:
Current File Browser:

Pictured solution: