💭 Rethink validation pattern for names

Issue was raised her in regards to team contact person: #577

# Description

We've occurrences of the concept of a name (not "username"):
- `KwTenantModel` `contactPerson`
- `TeamModel` `contactPerson`

The validation for these names is currently too strict. e.g. it does not allow Umlaute (ä, ü etc).

It seems (*) that we're not using the concepts of `firstname` `lastname` - which also can be tricky, because this would assume there's always a firstname/lastname and often also comes with the assumption that `lastname` equals to `familiyname` etc.

(*) as far as I could see in a quick skim, backend please jump in and update if that's not tree 🙈

## Notes
Validating names can be tricky (min. character, no special characters etc) and we should look into how we define the concept of "names" in Klaw and if there's room for improvement, also in regards to allowed pattern.