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/SessionDropdown.js

@@ -0,0 +1,54 @@
+import avatar from '../../common/helpers/avatar';
+import username from '../../common/helpers/username';
+import Dropdown from '../../common/components/Dropdown';
+import Button from '../../common/components/Button';
+import ItemList from '../../common/utils/ItemList';
+
+/**
+ * The `SessionDropdown` component shows a button with the current user's
+ * avatar/name, with a dropdown of session controls.
+ */
+export default class SessionDropdown extends Dropdown {
+  static initProps(props) {
+    super.initProps(props);
+
+    props.className = 'SessionDropdown';
+    props.buttonClassName = 'Button Button--user Button--flat';
+    props.menuClassName = 'Dropdown-menu--right';
+  }
+
+  view() {
+    this.props.children = this.items().toArray();
+
+    return super.view();
+  }
+
+  getButtonContent() {
+    const user = app.session.user;
+
+    return [
+      avatar(user), ' ',
+      <span className="Button-label">{username(user)}</span>
+    ];
+  }
+
+  /**
+   * Build an item list for the contents of the dropdown menu.
+   *
+   * @return {ItemList}
+   */
+  items() {
+    const items = new ItemList();
+
+    items.add('logOut',
+      Button.component({
+        icon: 'fas fa-sign-out-alt',
+        children: app.translator.trans('core.admin.header.log_out_button'),
+        onclick: app.session.logout.bind(app.session)
+      }),
+      -100
+    );
+
+    return items;
+  }
+}