Support Jupyter Content API

[oeway]

Jupyter has a standard content management API which will also been supported by Jupyter server. There are also ongoing work to support more backends with JupyterFS.

So it would be nice to support the content API directly. Either from the backend (in the form of a jupyter server extension) or build an frontend adapter for imjoy-elfinder by making an transport adapter (for elfinder connector api).

One issue I see is that, jupyter content api do not allow direct content serving (reported here).

Another limitation, at least for the classic notebook server, the content API is limited to access the notebook work directory for security considerations, we cannot access manage the entire volume or multiple volume (for windows) if the user doesn't start the server from root. However, I am not sure how jupyter server set its working directory. @Zsailer Could you perhaps comment on this?