[feature]: Customize the e-mail templates

[LarsJaensch]

Is there an existing issue for this?

• [X] I have searched the existing issues

Summary

It would be nice if you could customize the messages that the system sends to users. So the header, the footer and the content.

Why should this be worked on?

This would move more in the direction of whitelabel and users could customize it according to their personal needs

https://discord.com/channels/1031547764020084846/1227262578955386950/1232598952596213760