Linked Items BE: Add RelatedWorkItemLink model

In order to create and store the new link records we need to add the following functionality:

• Add a new model WorkItems:: RelatedWorkItemLink that includes the IssuableLink concern and uses the existing issue_links table.

See PoC implementation as an example: !124328 (4e06b3ac) (note that this MR is using WorkItems::Link but Link might be too generic so RelatedWorkItemLink was proposed instead. See discussion: #378059 (comment 1448890003))

• Add a validation check so items cannot be created if already related as parent and child