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.
Reviewed By: btrahan
Subscribers: eadler, joshuaspence, epriestley
Maniphest Tasks: T8726
Differential Revision: https://secure.phabricator.com/D13497