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/SelectDropdown.js

@@ -0,0 +1,34 @@
+import Dropdown from './Dropdown';
+import icon from '../helpers/icon';
+
+/**
+ * The `SelectDropdown` component is the same as a `Dropdown`, except the toggle
+ * button's label is set as the label of the first child which has a truthy
+ * `active` prop.
+ *
+ * ### Props
+ *
+ * - `caretIcon`
+ * - `defaultLabel`
+ */
+export default class SelectDropdown extends Dropdown {
+  static initProps(props) {
+    props.caretIcon = typeof props.caretIcon !== 'undefined' ? props.caretIcon : 'fas fa-sort';
+
+    super.initProps(props);
+
+    props.className += ' Dropdown--select';
+  }
+
+  getButtonContent() {
+    const activeChild = this.props.children.filter(child => child.props.active)[0];
+    let label = activeChild && activeChild.props.children || this.props.defaultLabel;
+
+    if (label instanceof Array) label = label[0];
+
+    return [
+      <span className="Button-label">{label}</span>,
+      icon(this.props.caretIcon, {className: 'Button-caret'})
+    ];
+  }
+}