Make CSS processing the same in dev and prod

[c-w]

This pull request is a revival of #8148 given that react-scripts is now on webpack 5 and MiniCssExtractPlugin now supports hot reloading.

With this pull request, we can make CSS processing the same between dev and prod builds which helps in avoiding hard to debug issues where some CSS works fine locally but doesn't work when deployed due to ordering conflicts or similar. It's a problem I ran into recently and currently have to work-around by manually patching react-scripts in postinstall.

Tagging @andriijas @elektronik2k5 @vatz88 who participated in the original pull request.

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