Sass loader

Created by: Fabianopb

Opt-in support for SASS and CSS-modules using SASS files added, using both .scss and .sass extensions. This PR:

• Installs sass-loader in the react-scripts package
• Adds webpack configuration to allow importing scss/sass directly into components during development, supporting also the use of CSS-modules
• Adds webpack configuration for bundling scss/sass files into the optimized production build, supporting also the use of CSS-modules
• CSS-modules in SASS are supported in a similar way than using purely CSS, which in this case is naming SASS files as .module.scss or .module.sass.

~No automated test just yet (coming in a couple days)! However I've manually tried the e2e flow using both scss and sass extensions, with or without modules, and it worked just fine.~~ edit: ✅ Unit and integration tests added!

This PR depends on https://github.com/webpack-contrib/sass-loader/pull/533 to work properly. ✅ Merged and integrated!

I'll be happy to get comments! :)

edit: @andriijas mentioned that we should use react-dev-utils/getCSSModuleLocalIdent when https://github.com/facebook/create-react-app/pull/4192 is merged, for the localIdentName. ✅ Merged and integrated!