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
Why do you use that condition within the getter? Why not reuse the pattern from above (https://github.com/opf/openproject/pull/2239/files#diff-da421ae54f2201015bc50791b17585efR54).

Will the link be displayed in either case?