[WIP] 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.
@myabc @manwithtwowatches @hschink Could smb please take a look at this? I am not entirely sure how the autocomplete works. I've hardcoded relation id value here as `relates`. The problem is that there are two inputs with same id now. I am sure this will have some really simple solution. I will be very thankful for some hint from someone who has worked with autocomplete before.