No RTE in FE - missing folder /assets/tinymce4/js/skins/lightgray

Looks like your tinyMCE configuration is missing the setting
```js
    skin: 'contao',
```
and so it tries to fetch a default but not available skin 'lightgray'.

https://github.com/DanielGausi/Contao-CalendarEditor/blob/master/src/Resources/contao/tinyMCE/tinyFrontendMinimal.php#L30-L36

BTW is it possible to use a custome configuration file? And how?