Add filter by object ability to flag query

Summary: See title. Fixes T1809.

Test Plan:
verified each type that has flaggable interface still can be flagged

verified that new custom query filter works

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T1809

Differential Revision: https://secure.phabricator.com/D7392