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
- https://community.openproject.org/work_packages/17432
- https://community.openproject.org/work_packages/17436

- [x] Path service
@hschink very minor, but `api_v3_paths.user(id) if id` looks more 'Ruby-ish' to me than the ternary operator here.