Feature/changing parent on relations tab

* https://www.openproject.org/projects/finnlabs-team-2/work_packages/14055/overview

I've implemented the basic behaviour. It slightly differs from the specification in the WP 14055, but it is more consistent with rest of the relations.
This was something I've been thinking about and I wasn't quite sure how to proceed but if it was the intention from beginnig I will just overwrite `canAddRelation` method here. Thanks.