Additional info per day

In eCamp v2, it was possible to store some additional info per day, and that info was then printed in the day overview. This issue is for gathering requirements, so we can design a feature that really meets the needs of the users, rather than just copying features from eCamp v2.

## Requirements
- [ ] Storing a day title / "Tagesthema", i.e. like the camp motto but per day, probably a short text (requested by Daniel Hermann via email)
- [ ] Storing daily notes, i.e. a rich text field (could be used for daily story context, although we should consider integrating the story overview instead here)
- [ ] Additional types of day responsibilities (this is tracked in #3888)

## Open questions
- Where in the UI would this info be editable?
- How do we avoid adding more and more fields for more and more specific requests? How do we implement a general, flexible feature that supports a wide range of feature requests?

No files, tests, or entry points are named. Start by reviewing the existing day overview and collecting answers to the listed UI, flexibility, and scope questions; the work is done when agreed requirements and a feature design are documented.

Written by the indexing model from the issue text.