Clarify and extend additional fields
The current "additoinal fields" feature related to additoinal fields on the relation between persons and groups – i.e. they are fields that can be filled-in for one pair of person and group.
This is not clear from the UI – it simply says "Additional fields", without any clarification about that it does.
While cleaning this up, we should extend the feature to have:
- Additional fields for a person (independent of a group)
- Additional fields for a group (independent of a person)
- Additional fields for a person in a group