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