UIEvents - add support for "subscribers property"

Summary: Fixes T3487 and reduces a bit of code duplication.

Test Plan: viewed legalpad, macro, and pholio and saw proper subscriber properties

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T3487

Differential Revision: https://secure.phabricator.com/D6383