Support registering as an extension for jupyter_server (alongside jupyter notebook)

[consideRatio]

@damianavila pointed me to some relevant discussion that may or may not relate to #176, but should probably be addressed no matter what.

In this documentation for jupyter_server, there are notes on how to make a classical jupyter notebook extension register itself with jupyter_server.

https://jupyter-server.readthedocs.io/en/latest/developers/extensions.html#migrating-an-extension-to-use-jupyter-server

Based on the current setup.py file, I conclude that nbgitpuller currently isn't a extension compatible with both the classical notebook server and the new jupyter_server.

nbgitpuller/setup.py

Lines 25 to 27 in 207e3b2

	data_files=[
	('etc/jupyter/jupyter_notebook_config.d', ['nbgitpuller/etc/nbgitpuller.json'])
	],

Suggested action points

• Follow the jupyter_server docs on registering an extension and register nbgitpuller as a jupyter_server extension alongside the jupyter notebook extension.
• Update documentation in the README.md about this