Validate concordance.persons
At the moment a user can add or modify concordances if he/she moves a person to another concordance. To do so, he/she can just send the new Concordance json to the server. The server does not forbid to send new persons in this request concordance (unknown to the system) or to modify persons in the system. This helps us to have the ability to edit the data in a concordance, but we should at least check it better!