Add webpack prod config

[gjmooney]

Continuing @jtpio's work in #7354

Adds HtmlWebpackPlugin to inject cache-busting script tag.

Fixes #7373
Fixes #7221

[github-actions]

👈 Launch a Binder on branch gjmooney/notebook/webpack_prod

[jtpio]

Thanks @gjmooney for picking this up!

Checking with the assets built by the releaser on CI, the ?v= cache busting parameter now seems to be inlined correctly:

While it was not the case until now, for example when testing with notebook 7.2.2:

So it looks good, also since it follows what JupyterLab does: jupyterlab/jupyterlab#9776

[jtpio]

Thanks!

[jtpio]

Cutting a new beta with this change now.