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

For me **from** and **to** looks more agnostic to the underlying database-schema, so I would prefer this instead of the id of the relation. This way it is possible to "guess" the link for relations from one WP to another. While that is not strictly neccessary, it is a nice feature.

@mtakac Is there a specific reason you do **not** want to use 2 WP-ids? (i.e. any counter argument)

@myabc I am assuming that the endpoint `/work_packages/1/relations/2` is giving me **all** the relations between WP 1 and WP 2 (that means a list of WPs). Is that assumption correct?