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
@ulferts I guess everyone that can view work packages is also able to see the users of a project, right? Because we basically allow the client to list (most?) users of the project...