When Herald changes object subscribers, always hide the feed story

Summary:
Fixes T8952. These feed stories are not interesting and tend to be generated as collateral damage when a non-story update is made to an old task and someone has a "subscribe me" Herald rule.

Also clean up some of the Herald field/condition indexing behavior slightly.

Test Plan: Wrote a "Subscribe X" herald rule, made a trivial update to a task. Before: low-value feed story; after: no feed story.

Maniphest Tasks: T8952

Differential Revision: https://secure.phabricator.com/D20797