R1:7a315780b4fe

When using the "Close as Duplicate" relationship action, limit the UI to 1 task

Summary:
Ref T4788. When closing a task as a duplicate of another task, you can only select one task, since it doesn't really make sense to merge one task into several other tasks (this operation is //possible//, but probably not what anyone ever wants to do, I think?).

Make the UI understand this: after you select a task, disable all of the "select" buttons in the UI to make this clear.

Test Plan:
- Used "Close as…