Migrate server extensions to jupyter_server

[jgwerner]

Overview

Migrate the nbgrader server extensions to support both the Jupyter classic notebook server and the jupyter_server. This would be one of the first steps to allow nbgrader to work with the newer version of Jupyter NB Classic or with JupyterLab.

Tasks

• Add new reference name for load_jupyter_server_extension with _load_jupyter_server_extension
• Update readme/docs
• Update tests

I don't see any nbgrader extension files located in the /path/to/etc/jupyter/jupyter_server_config.d directory so it doesn't look like the setup.py needs to be updated, but perhaps I'm missing something.