Refactor repo with new jupyterlab extension cookiecutter

[ryantam626]

Big bang commit because it's harder to work out what a bitesized commit should look like, there is simply way too many changes needed.

• Partial revert to rtd docs;
• Add dockerised dev env with hatch support (kinda - need to work out how to use hatch properly later);
• Port most logic to new jupyterlab extension cookiecutter;
• Remove server version check, it's pretty hard to have a mismatching version now due to the entire extension to be packaged with a pypi package;
• Clean up some labextension code;

[ryantam626]

yolo.