Reduce strength of Herald and user subscription stories
Fixes T8952. When Herald changes subscribers, it is zzzzz very boring.
When users change subscribers, it is still super boring (more boring than a merge, for example).
Test Plan: Viewed feed, saw fewer Herald stories.
Reviewed By: chad
Maniphest Tasks: T8952
Differential Revision: https://secure.phabricator.com/D15774