Add "Subscribers" to Maniphest application search

Summary: Fixes T3883. This is already supported in the query, expose it in the UI.

Test Plan: Ran some queries with and without subscribers.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3883

Differential Revision: https://secure.phabricator.com/D7161