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/forum/components/ComposerButton.js

@@ -0,0 +1,13 @@
+import Button from '../../common/components/Button';
+
+/**
+ * The `ComposerButton` component displays a button suitable for the composer
+ * controls.
+ */
+export default class ComposerButton extends Button {
+  static initProps(props) {
+    super.initProps(props);
+
+    props.className = props.className || 'Button Button--icon Button--link';
+  }
+}