Make the installation use the notebook 5.3 nbextension conf.d installation approach

[jasongrout]

This would make it so that we don't have to use the post-link script in the conda package, and the pip package would not need to also do jupyter nbextension install ....

See https://github.com/jupyter-widgets/widget-ts-cookiecutter/pull/9/files for an example of how to do this.

See jupyter/notebook#3116 for the notebook PR, released in notebook 5.3.

[jasongrout]

Another example of how to do this is widgetti/ipyvolume@415f0fb

[blink1073]

Happy to help with this at the workshop.

[jasongrout]

Thanks. I've been accumulating a lot of "good first issue"s particularly for people wanting to tackle things at the workshop.

[maartenbreddels]

working on it :)

[maartenbreddels]

or maybe I shouldn't, let me know.

[jasongrout]

I think it's fine. We definitely have enough "good first issue"s to go around.

[jasongrout]

(but if you have other stuff to do, and can wait a few days, you can do it with a new contributor next week and help them come up to speed...)

[jasongrout]

Fixed in #1911