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/Switch.js

@@ -0,0 +1,17 @@
+import Checkbox from './Checkbox';
+
+/**
+ * The `Switch` component is a `Checkbox`, but with a switch display instead of
+ * a tick/cross one.
+ */
+export default class Switch extends Checkbox {
+  static initProps(props) {
+    super.initProps(props);
+
+    props.className = (props.className || '') + ' Checkbox--switch';
+  }
+
+  getDisplay() {
+    return this.loading ? super.getDisplay() : '';
+  }
+}