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