Provide an alternate design for work package relations

https://www.openproject.org/work_packages/12233

See also discussion:
* https://github.com/opf/openproject/commit/91ba7ba6fba688dc61b09215ae39813e3a79149f
* https://www.openproject.org/work_packages/11676
* https://github.com/opf/openproject/pull/1542

@mtakac after giving it some thought, I find this less ambiguous.