[wip] Provide an alternate design for work package relations


See also discussion:
* https://github.com/opf/openproject/commit/91ba7ba6fba688dc61b09215ae39813e3a79149f
* https://www.openproject.org/work_packages/11676
* https://github.com/opf/openproject/pull/1542

there is no ```self``` link here. Would the relationship be nested within a work package? Otherwise there would be the need for a second work package in here as well.

Otherwise, I am fine with it.