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
??? I don't get it. A comment please.

My understanding is, that you with this rely on somebody having called valid before, which sounds bad.