Work items: Simplify changing relationship type in linked items

Problem to solve

When an item is already linked in "Linked items", users can change the relationship type by either dragging the item into another list (requires other lists present) or removing the item and relinking it. Trying to add this as an existing item shows "No matches" which is unclear, since the item exists. In general it is less clear than ideal how to change an item relationship.

Proposal

MVC1: Allow selecting existing items when adding, and just remove the existing link if different, e.g. if #1 is linked as "related", a user can add existing item and link #1 as a blocking relationship. This will work the same as dragging #1 from "related" to "blocking".

Later iterations: Consider ways to allow users to directly change the relationship type from the linked item that can always be utilized