Use the new "CurtainObjectRefList" UI element for subscribers

Summary:
Depends on D20966. Ref T13486. Curtains currently render subscribers in a plain text list, but the new ref list element is a good fit for this.

Also, improve the sorting and ordering behavior.

This makes the subscriber list take up a bit more space, but it should make it a lot easier to read at a glance.

Test Plan: Viewed object subscriber lists at varying limits and subscriber counts, saw sensible subscriber lists.

Maniphest Tasks: T13486

Differential Revision: https://secure.phabricator.com/D20967