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/HeaderSecondary.js

@@ -0,0 +1,37 @@
+import Component from '../../common/Component';
+import SessionDropdown from './SessionDropdown';
+import ItemList from '../../common/utils/ItemList';
+import listItems from '../../common/helpers/listItems';
+
+/**
+ * The `HeaderSecondary` component displays secondary header controls.
+ */
+export default class HeaderSecondary extends Component {
+  view() {
+    return (
+      <ul className="Header-controls">
+        {listItems(this.items().toArray())}
+      </ul>
+    );
+  }
+
+  config(isInitialized, context) {
+    // Since this component is 'above' the content of the page (that is, it is a
+    // part of the global UI that persists between routes), we will flag the DOM
+    // to be retained across route changes.
+    context.retain = true;
+  }
+
+  /**
+   * Build an item list for the controls.
+   *
+   * @return {ItemList}
+   */
+  items() {
+    const items = new ItemList();
+
+    items.add('session', SessionDropdown.component());
+
+    return items;
+  }
+}