Convert env() to config()

Laravel [recommends][1] only using `env()` within configuration files and using `config()` everywhere else. Doing so allows you to improve performance by running `php artisan config:cache`.

[1]: https://laravel.com/docs/5.7/configuration#configuration-caching

app/Http/Controllers/Auth/RegisterController.php

@@ -232,7 +232,7 @@ class RegisterController extends Controller
         app('smarty.view')->assign('invite_code_query', $this->inviteCodeQuery);
         $theme = Settings::settingValue('site.main.style');
 
-        $nocaptcha = env('NOCAPTCHA_ENABLED');
+        $nocaptcha = config('settings.nocaptcha_enabled');
 
         $meta_title = 'Register';
         $meta_keywords = 'register,signup,registration';