Upgrade front end dependencies

Mostly in order to get rid of deprecated deps, and upgrade vulnerable ones.

  - Upgrade webpack from 3.x to 4.x
  - Moved babel package to main repo
  - Replaced deprecated extract-text-webpack-plugin with extract-text-webpack-plugin
  - Replaced deprecated babel-minify-webpack-plugin with terser-webpack-plugin
  - Replaced deprecated node-sass with (dart) sass package
  - Replaced deprecated sass-lint with stylelint (the rules might be a bit different

Related to #1531: trivy doesn't raise any more issue