fix feed story publishing for audit comments

Summary: we need to make sure we should publish to the auditor in a given audit request. write some custom logic for this as it is subtly different than other things like CC.

Test Plan: repro from T2087 produced expected results. further, former auditors who resigned did not get feed stories published.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2087

Differential Revision: https://secure.phabricator.com/D3987