Minify each JS file individually, caching the result

This means that the expensive minification process will only be run for a file if it hasn't before. Greatly speeds up extension enabling/disabling.

Also:
- Don't check file last modification times in production for a bit of extra perf.
- Only flush CSS when theme settings are changed. This speeds up the page reload a bit.

src/Forum/Controller/ClientController.php

@@ -11,10 +11,12 @@
 namespace Flarum\Forum\Controller;
 
 use Flarum\Api\Client;
+use Flarum\Formatter\Formatter;
 use Flarum\Foundation\Application;
 use Flarum\Settings\SettingsRepository;
 use Flarum\Locale\LocaleManager;
 use Flarum\Http\Controller\AbstractClientController;
+use Illuminate\Contracts\Cache\Repository;
 use Illuminate\Contracts\Events\Dispatcher;
 
 class ClientController extends AbstractClientController
@@ -29,6 +31,11 @@ class ClientController extends AbstractClientController
      */
     protected $translationKeys = ['core.forum', 'core.lib'];
 
+    /**
+     * @var Formatter
+     */
+    protected $formatter;
+
     /**
      * {@inheritdoc}
      */
@@ -37,11 +44,14 @@ class ClientController extends AbstractClientController
         Client $api,
         LocaleManager $locales,
         SettingsRepository $settings,
-        Dispatcher $events
+        Dispatcher $events,
+        Repository $cache,
+        Formatter $formatter
     ) {
-        parent::__construct($app, $api, $locales, $settings, $events);
+        parent::__construct($app, $api, $locales, $settings, $events, $cache);
 
         $this->layout = __DIR__.'/../../../views/forum.blade.php';
+        $this->formatter = $formatter;
     }
 
     /**
@@ -52,7 +62,7 @@ class ClientController extends AbstractClientController
         $assets = parent::getAssets();
 
         $assets->addJs(function () {
-            return $this->app->make('flarum.formatter')->getJs();
+            return $this->formatter->getJs();
         });
 
         return $assets;