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