Update webpack 3.1.0 et al

resources/assets/build/helpers/hmr-client.js

@@ -0,0 +1,7 @@
+const hotMiddlewareScript = require('webpack-hot-middleware/client?noInfo=true&timeout=20000&reload=true');
+
+hotMiddlewareScript.subscribe(event => {
+  if (event.action === 'reload') {
+    window.location.reload();
+  }
+});

resources/assets/build/helpers/public-path.js

@@ -0,0 +1,7 @@
+/* eslint-env browser */
+/* globals SAGE_DIST_PATH */
+
+/** Dynamically set absolute public path from current protocol and host */
+if (SAGE_DIST_PATH) {
+  __webpack_public_path__ = SAGE_DIST_PATH; // eslint-disable-line no-undef, camelcase
+}