Add user settings to frontend, use them to configure FSContentsManager on the backend

[telamonian]

fixes: #22

In this PR I will:

• create user settings for jupyter-fs

  • top level specs field that holds a list of spec (as in "filesystem specification") objects:

    "specs": [
      {
        "name": "osfs_test";
        "url": "osfs:///Users/foo/whatever";
      },
      ...
    ]

• ship list of resources to backend

• dynamically initialize multiple fs contents instances on the server backend based on the list of resources provided by the frontend

• ship list of initialized resources to frontend

• dynamically initalize multiple FileTree on the frontend based the list of initalized resources provided by the backend

• glue all of the above together

• work out various kinks and bugs in FileTree/improve FileTree UI

• update the README to reflect all of the above

[telamonian]

It's done!

The final version of this PR covers everything mentioned in the top post, and a bit more. Lots of misc fixes and improvements.