Webpack (#4)

See https://github.com/flarum/core/pull/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"
* Restructure `less` directory

extensions/statistics/src/Listener/AddClientAssets.php

@@ -1,40 +0,0 @@
-<?php
-
-/*
- * This file is part of Flarum.
- *
- * (c) Toby Zerner <toby.zerner@gmail.com>
- *
- * For the full copyright and license information, please view the LICENSE
- * file that was distributed with this source code.
- */
-
-namespace Flarum\Statistics\Listener;
-
-use Flarum\Frontend\Event\Rendering;
-use Illuminate\Contracts\Events\Dispatcher;
-
-class AddClientAssets
-{
-    /**
-     * @param Dispatcher $events
-     */
-    public function subscribe(Dispatcher $events)
-    {
-        $events->listen(Rendering::class, [$this, 'addAssets']);
-    }
-
-    /**
-     * @param Rendering $event
-     */
-    public function addAssets(Rendering $event)
-    {
-        if ($event->isAdmin()) {
-            $event->addAssets([
-                __DIR__.'/../../js/admin/dist/extension.js',
-                __DIR__.'/../../less/admin/extension.less'
-            ]);
-            $event->addBootstrapper('flarum/statistics/main');
-        }
-    }
-}