[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

@NobodysNightmare it was deliberate! :-) titles are [OPTIONAL](http://tools.ietf.org/html/draft-kelly-json-hal-06#section-5.7). I think we should only include them if they actually contain meaningful information.