Move Herald rule querying into HeraldRuleQuery
Summary: Ref T2769. The HeraldRule class has some query logic; move it into HeraldRuleQuery. Also some minor cleanup.
Test Plan: Ran test console, created a new revision, used reparse.php --herald. Verified rules triggered correctly.
Reviewers: btrahan
Reviewed By: btrahan
CC: aran
Maniphest Tasks: T2769
Differential Revision: https://secure.phabricator.com/D6689