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 Duplicate", only allowed to select 1 task.
• Used other editors like "Merge Duplicates In", allowed to select lots of tasks.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4788

Differential Revision: https://secure.phabricator.com/D16203