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
