16972 add assignee responsible to wp form

[``* `#16972` Support assignee/responsible property on work package forms and PATCH endpoint``](https://community.openproject.org/work_packages/16972)

Includes https://community.openproject.org/work_packages/17426
This error message is not very good. If we can get rid of that magical -1, we could focus on what the list of available values provides us and return that the user is not within the list of possible assignees/responsibles.