Reduce strength of Herald and user subscription stories

Summary:
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.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T8952

Differential Revision: https://secure.phabricator.com/D15774