Allow multiple custom checkout message panes

Problem/Motivation

Allow multiple custom checkout message panes. Not yet sure, how to do that best architectually, but I think it would be helpful.

Perhaps we might just add a settings page, where the number of panes can be defined and added by loop then?

Happy to see feedback and MRs.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes