Update backend configuration for including and excluding buckets

[DenisaCG]

Follow-up to #89.

Update backend configuration to allow users to specify a list of drives to include or exclude from the drives filebrowser upon starting JupyterLab.

The config now contains two new fields, excluded_drives and included_drives, through which users can define a smaller set of drives they want to launch JupyterLab with. The given list of drives will be used to populate the UI for the drives manager linked in the above PR. As such, the user will still have control over their full set of buckets and will be able to add drives back to the filebrowser from the excluded list, as well as remove others from the included list.

Although both fields are present, only one of them is needed when configuring the drives list - they are just meant to give users a way to easily define the shortest list (e.g.: include only these three drives and exclude all else, opposed to exclude this long list of drives).

[github-actions]

👈 Launch a Binder on branch DenisaCG/jupyter-drives/include_exclude_config

[afshin]

Thank you! This looks good.

If we get some validation from people who deploy this asking for some refinement we can do a follow-on PR, but this looks complete to me.