Rework extension bootstrapping

System JS modules don't execute when they're registered, so we need to
import them explicitly. While we're at it, we may as well make the
locale bootstrapper a module too.

stubs/extension/src/Extension.php

@@ -0,0 +1,27 @@
+<?php namespace {{namespace}};
+
+use Flarum\Support\Extension;
+use Illuminate\Events\Dispatcher;
+
+class Extension extends BaseExtension
+{
+    /**
+     * Bootstrap the application events.
+     *
+     * @return void
+     */
+    public function boot(Dispatcher $events)
+    {
+        $events->subscribe('{{namespace}}\Listeners\AddClientAssets');
+    }
+
+    /**
+     * Register the service provider.
+     *
+     * @return void
+     */
+    public function register()
+    {
+        //
+    }
+}