[JEP 0028] Add namespacing to `static` endpoints and REST API urls.

[Zsailer]

From JEP 0028 (Jupyter Server):

Add namespacing to static endpoints and REST API urls.

Currently, the notebook tornado application serves all static files underneath the /static/ prefix. Jupyter server will add namespacing under the static url and extension REST API urls. Each extension will serve their static files under the /static/<extension-name> prefix and their API handlers behind a /extension/api/<extension-name> prefix.

For example, the classic notebook server extension will add static handlers that reroute requests to the /static/notebook/ endpoints.

A new ExtensionHandler class will be available in jupyter_server.extensions.handlers. This class inherits JupyterHandler. It handles the boilerplate code to reroute requests extension's namespaced static and REST API endpoints.

Preliminary experimental work resides in the jupyter_server_extension repository.

Opening for further discussion.

[Zsailer]

Closing, since this was added in Jupyter Server >= 1.x