Inlines for custom herald actions
Summary: Ref D8784. Didn't see all of the inlines before hitting arc land. This fixes up the issues raised (and makes all the code nicer).
Test Plan: Made sure custom actions only appear for appropriate adapters and checked to ensure that they triggered correctly.
Reviewers: #blessed_reviewers, epriestley
Reviewed By: #blessed_reviewers, epriestley
Subscribers: edutibau, ite-klass, epriestley, Korvin
Differential Revision: https://secure.phabricator.com/D9796