Improve usage of Fluid templates

Enable usage of pure Fluid templates to avoid Markdown parsing.
Enable usage of Fluid layouts.
Correct a wrong type declaration.
You are right that the Backend is calling a FE URL to give Fluid the adequate context, so that it can correctly render the URIs. However, it will not go into the ``renderFrontendMode`` as one might suppose but rather into the [Message Renderer Controller](https://github.com/fudriot/messenger/blob/master/Classes/Controller/MessageRendererController.php#L40). After double checking the code, it looks the Backend will not process the Markdown at all! Making a note to myself to improve. On the long term, my plan is to hook into the Vidi module of FE / BE Users so that a BE Editor can send messages from there...