Stop evaluating Herald rules when writing "someone mentioned this somewhere else." transactions

Summary: Ref T13114. See PHI510. Firing Herald on mentioned objects tends to feel arbitrary and can substantially slow down edits which mention many objects.

Test Plan: Mentioned tasks on other tasks; verified that the normal path is hit normally, the new Herald-free path is hit on the mentioned object, and both still work fine and show up in the timeline.

Maniphest Tasks: T13114

Differential Revision: https://secure.phabricator.com/D19263