Allow transactions to specialize their mail headers for diff sections
Summary: Ref T7643. When we send mail about a change to a package description, allow it to say "CHANGES TO PACKAGE DESCRIPTION" instead of "EDIT DETAILS". Smooth!
Test Plan:
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T7643
Differential Revision: https://secure.phabricator.com/D16818