Set related links foreign keys and remove service

Nicolas Dularrequested to merge
nd/remove-related-link-service-usage into master

What does this MR do and why?

Part of #497765 (closed)

Set related links foreign keys and remove service

This removes the call from the WorkItems service to the RelatedEpicLinks service and replaces it by creating the ActiveRecord directly.

This allows us to set the foreign key on the related_epic_link to the work_item_link.

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