replace extract-text-webpack-plugin (deprecated) by mini-css-extract-plugin. Try to make webpack-dev-server work again on the project
$font-family-sans-serif: "Open Sans", sans-serif;
$icon-font-path: "bootstrap-sass/assets/fonts/bootstrap/";
@import "~bootstrap-sass/assets/stylesheets/bootstrap";
$fa-font-path: "font-awesome/fonts";
@import "~font-awesome/scss/font-awesome";
@import "~intro.js/introjs";
@import "iconolab";