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: {F1909417}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T7643

Differential Revision: https://secure.phabricator.com/D16818