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 ``if`` statement will prohibit the rendering of this property. But we want to have it even if no assignee/responsible is set.

The ``if`` in line 54 is plain wrong: I'll remove it.