R1:a4ba7daf9009

Add transitional support for mail tags to Differential on EditEngine

Summary: Ref T11114. Although I plan to rewrite this system eventually (T10448) it's easy enough to punt for now.

Test Plan: punt

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11114

Differential Revision: https://secure.phabricator.com/D17119