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
The necessary permission for this is not specified @NobodysNightmare. I would opt for :view_work_packages as this is the intended audience. :view_project has the additional disadvantage that it is a public permission meaning everybody having any permission within the project also has this one.