Replace creation of User entity with a more dynamic, configurable setup step

[sleidig]

The setup script currently creates a User entity in a very fixed way. Since we have moved on and allow a user account to link with any entity type, this needs an update to allow such handling.

See: https://github.com/Aam-Digital/ndb-setup/blob/13922c82dc07fdfe23186ef274e2de4d5d746094/interactive_setup.sh#L164-L172

Possible approaches:

• don't create any entity for the user during setup (and maybe accept a uuid to link to a separately created entity? or how would a user be able to link their initial account with an entity they create later?)
• make this more flexible / configurable, what entity is created
• add a UI to the frontend that asks a user to create an entity for their account after login, if the account is not linked to any yet