Fix loading child items that are also related

What does this MR do and why?

Fixes #478524 (closed)

Fix loading child items that are also related

This fixes a bug when a work item is a child item and a related item at the same time.

Changelog: fixed

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Before	After
bug	fixed

How to set up and validate locally

Enable the following feature flags: work_item_epics, work_item_epics_rollout

1. Create an epic
2. Add a child epic
3. Disable the validation by applying the following diff:

   diff --git a/app/models/concerns/linkable_item.rb b/app/models/concerns/linkable_item.rb
    index c91e3615ba75..412ca600fef5 100644
    --- a/app/models/concerns/linkable_item.rb
    +++ b/app/models/concerns/linkable_item.rb
    @@ -24,12 +24,12 @@ module LinkableItem
         private
     
         def check_existing_parent_link
    -      return unless source && target
    +      # return unless source && target
     
    -      existing_relation = WorkItems::ParentLink.for_parents([source, target]).for_children([source, target])
    -      return if existing_relation.none?
    +      # existing_relation = WorkItems::ParentLink.for_parents([source, target]).for_children([source, target])
    +      # return if existing_relation.none?
     
    -      errors.add(:source, format(_('is a parent or child of this %{item}'), item: self.class.issuable_name))
    +      # errors.add(:source, format(_('is a parent or child of this %{item}'), item: self.class.issuable_name))
         end
       end
     end

4. Add the same child epic now as a linked epic
5. Add 30 more child items so the child hierarchy widget needs to paginate
6. Reload the page to see the pagination
7. Click "Load more" -> before this change, an error appeared, with this fix the next child items get loaded