Update to webpack 5, remove gulp

Co-Authored-By: Mark McDowall <markus101@users.noreply.github.com>
(cherry picked from commit 4ef2174226a0210f756f180dded8567d659589e2)
(cherry picked from commit 06730363c0fe1e2bf376122f50943fbf90ab60e8)
(cherry picked from commit 533f1a61d52cc5d2b7c879de4b2291dbad833cbf)

frontend/build/webpack/css-variables-loader.js

@@ -0,0 +1,11 @@
+const loaderUtils = require('loader-utils');
+
+module.exports = function cssVariablesLoader(source) {
+  const options = loaderUtils.getOptions(this);
+
+  options.cssVarsFiles.forEach((cssVarsFile) => {
+    this.addDependency(cssVarsFile);
+  });
+
+  return source;
+};