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/common/components/Separator.js

@@ -0,0 +1,14 @@
+import Component from '../Component';
+
+/**
+ * The `Separator` component defines a menu separator item.
+ */
+class Separator extends Component {
+  view() {
+    return <li className="Dropdown-separator"/>;
+  }
+}
+
+Separator.isListItem = true;
+
+export default Separator;