Webpacker v6 -- Blocker to GA -- extract_css removal

[justin808]

Per my review of #2802 (review)

We need to do something about the removal of extract_css from config/webpacker.yml

AFAIK, if one wants to use HMR with CSS, with the webpack-dev-server, in development mode, then we want CSS to come from the regular JS Bundle, so:

1. We don't use the view helper for the CSS, since
2. We must not use the mini-css-extract-plugin

CC: @gauravtiwari

[justin808]

@gauravtiwari this may have changed in Webpack v5.

https://webpack.js.org/plugins/mini-css-extract-plugin/#hot-module-reloading-hmr

I'm investigating.

Did you test the React HMR with CSS changes?

[pedrofurtado]

@justin808 @gauravtiwari Maybe this issue can be related? --> #2832

[justin808]

Because css modules is probably the main way that hot loading of CSS works...

Per webpack-contrib/mini-css-extract-plugin#519 (comment), we need to use the style-loader for HMR.

[justin808]

See fix in #3031.