Allow dynamic adjustment of letter template

[rzo1]

The XSLT for password letter generation is currently included as part of the source code in the corresponding Docker image.

However, it would be good to be able to adapt the template dynamically via the frontend without having to perform a complete re-deployment with code changes.

Proposal:

• Add the possibility to configure the XSLT via the frontend
• Add the possibility to retrieve media from a location mapped to the docker container (as a first step). Next, we could also think of adding an upload mechanismn as a feature.