Make Herald action modularization more aggressive

Summary:
Ref T8726. Herald actions are technically sort-of modular already, but make them more aggressively modular similar to HeraldField.

I plan to obsolete and replace HeraldCustomAction.

Test Plan: Saw actions in nice groups; created and ran a "Do Nothing" action. Transcripts are a bit rough for now.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: joshuaspence, epriestley

Maniphest Tasks: T8726

Differential Revision: https://secure.phabricator.com/D13646