Server config: can't set contents_manager_class in jupyterfs.json

jupyterfs.json gets installed to <python-prefix>/etc/jupyter/jupyter_notebook_config.d/jupyterfs.json when you pip install the the jupyterfs Python package.

Ideally, jupyterfs.json should contain all of the server-side config needed to get jupyter-fs to work:

{
  "NotebookApp": {
    "contents_manager_class": "jupyterfs.metamanager.MetaManager",
    "nbserver_extensions": {
      "jupyterfs": true
    }
  }
}

However, in practice the "contents_manager_class" option gets ignored. Instead, "contents_manager_class" has to go in eg <python-prefix>/etc/jupyter/jupyter_notebook_config.json. Since I don’t want to accidentally overwrite an existing jupyter_notebook_config.json, this limits my options in terms of automating the install of jupyter-fs.

Currently, the only truly reliable option is to have users manually add the appropriate "contents_manager_class" to their server config, but that leads to all kinds of problems for the average user, and also isn’t automatic

[Zsailer]

In short, I don’t think this logic should live in the extension JSON. This JSON should really only be used to enable/disable extensions. Instead, server traits that the extensions must affect in order to work should be hardcoded in the extension package and imported through the _jupyter_server_extension_paths linker.

[Zsailer]

Cool. Yeah, that would be great! Check out the full conversation in jupyter/jupyter_server#207, because I added some code snippets with ideas for how we might discover+load config from server extensions.