JupyterLab3 support

[d-vasilev]

With the release of JupyterLab3 a new jupyter_server app has been introduced which is currently not supported by the package. More details available here https://stackoverflow.com/questions/67797152/what-is-the-difference-between-jupyter-notebook-and-jupyter-server

The proposed PR introduces support for both jupyter_notebook (existing one) and jupyter_server (new one). Which of them is used is detected by the web_app type provided to the extension loader. Handler class definitions and Kernel handlers updates are moved within do_load_jupyter_server_extension for maximum code reuse for the both cases.

Additionally jupyter_server configuration is added to setup.py