Implement an "only if the rule did not match last time" policy for Herald rules

Summary: Depends on D18927. Ref T13048. This implements a new policy which allows Herald rules to fire on some kinds of state changes.

Test Plan:
Wrote and tested rules with the new policy:

Also wrote and tested rules with the old policies:

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13048

Differential Revision: https://secure.phabricator.com/D18930