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/models/Forum.js

@@ -0,0 +1,7 @@
+import Model from '../Model';
+
+export default class Forum extends Model {
+  apiEndpoint() {
+    return '/';
+  }
+}