Make Herald custom field integration modular

Summary: Ref T8726. The existing implementation is largely made redundant by the newer modular implementation.

Test Plan: Created a custom field rule, set custom field to various values until it matched, saw effect and proper transcripts.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: eadler, joshuaspence, epriestley

Maniphest Tasks: T8726

Differential Revision: https://secure.phabricator.com/D13497