Webpack (#1367)

* Replace gulp with webpack and npm scripts for JS compilation
* Set up Travis CI to commit compiled JS
* Restructure `js` directory; only one instance of npm, forum/admin are "submodules"
* Refactor JS initializers into Application subclasses
* Maintain partial compatibility API (importing from absolute paths) for extensions
* Remove minification responsibility from PHP asset compiler
* Restructure `less` directory

js/src/admin/components/Page.js

@@ -0,0 +1,32 @@
+import Component from '../../common/Component';
+
+/**
+ * The `Page` component
+ *
+ * @abstract
+ */
+export default class Page extends Component {
+  init() {
+    app.previous = app.current;
+    app.current = this;
+
+    app.modal.close();
+
+    /**
+     * A class name to apply to the body while the route is active.
+     *
+     * @type {String}
+     */
+    this.bodyClass = '';
+  }
+
+  config(isInitialized, context) {
+    if (isInitialized) return;
+
+    if (this.bodyClass) {
+      $('#app').addClass(this.bodyClass);
+
+      context.onunload = () => $('#app').removeClass(this.bodyClass);
+    }
+  }
+}