Basic specification for patching of work package assignee and responsible

## OpenProject work packages

* https://community.openproject.org/work_packages/16986
* https://community.openproject.org/work_packages/16972
* https://community.openproject.org/work_packages/17426

## Changes

* specify assignee and responsible links for work packages (read and write)
* available assignees/responsibles moved to project level
* examples for collections refer to new collection specification

## Not in scope

* Filter collections using queries (e.g. filter by username)