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/Admin/AdminServiceProvider.php

@@ -71,7 +71,7 @@ class AdminServiceProvider extends AbstractServiceProvider
     {
         $this->app->make('events')->listen(SettingWasSet::class, function (SettingWasSet $event) {
             if (preg_match('/^theme_|^custom_less$/i', $event->key)) {
-                $this->flushAssets();
+                $this->getClientController()->flushCss();
             }
         });
     }
@@ -86,7 +86,14 @@ class AdminServiceProvider extends AbstractServiceProvider
 
     public function flushAssets()
     {
-        $action = $this->app->make('Flarum\Admin\Controller\ClientController');
-        $action->flushAssets();
+        $this->getClientController()->flushAssets();
+    }
+
+    /**
+     * @return \Flarum\Admin\Controller\ClientController
+     */
+    protected function getClientController()
+    {
+        return $this->app->make('Flarum\Admin\Controller\ClientController');
     }
 }

src/Admin/Controller/ClientController.php

@@ -13,6 +13,7 @@ namespace Flarum\Admin\Controller;
 use Flarum\Foundation\Application;
 use Flarum\Http\Controller\AbstractClientController as BaseClientController;
 use Flarum\Extension\ExtensionManager;
+use Illuminate\Contracts\Cache\Repository;
 use Illuminate\Contracts\Events\Dispatcher;
 use Psr\Http\Message\ServerRequestInterface as Request;
 use Flarum\Core\Permission;
@@ -47,9 +48,10 @@ class ClientController extends BaseClientController
         LocaleManager $locales,
         SettingsRepository $settings,
         Dispatcher $events,
+        Repository $cache,
         ExtensionManager $extensions
     ) {
-        BaseClientController::__construct($app, $apiClient, $locales, $settings, $events);
+        BaseClientController::__construct($app, $apiClient, $locales, $settings, $events, $cache);
 
         $this->layout = __DIR__.'/../../../views/admin.blade.php';
         $this->extensions = $extensions;