Move most Herald field configuration into dynamic Adapters
Summary: Ref T2769. Herald has a giant hard-coded list of fields. Primarily make these dynamic and adapter-based.
Test Plan: Viewed and edited Herald rules.
Reviewers: btrahan
Reviewed By: btrahan
CC: aran
Maniphest Tasks: T2769
Differential Revision: https://secure.phabricator.com/D6657