Don't run Herald when applying inverse edge transactions

Summary: Fixes T6727. Repro is: mention a task on another task, in a comment.

The inverse edge editor applying the "alincoln mentioned this in <other task>" transaction doesn't have enough data to execute Herald rules.

Just don't try to execute the rules, since they don't make much sesne from a product perspective and are tricky from a technical perspective.

Test Plan: Commented on `T1` with `T2` in comment body and a Herald rule that examines subscribers.

Reviewers: btrahan

NOTE: Cowboy committing this since any task mention fatals.