Use inline error when adding existing item to linked items

Summary

Currently when adding an item that already exists to Linked Items, a page-level error is shown. In the Epic Tree, this same scenario results in an inline validation error, which more directly connects the output of the action to the input.

Linked Items	Epic Child

Proposal

Use the same inline validation pattern within Linked Items, replacing the page level error.

This should apply to Linked items on Issues, Linked epics on Epics, and Linked incidents or issues on Incidents.