Restore "[Action]" mail subject lines to Differential/Diffusion

Summary: Ref T11114. Ref T10978. These hadn't made it over to EditEngine yet.

Test Plan:

• Took various actions on revisions and commits.
• Used bin/mail show-outbound --id ... to examine the "Vary Subject", saw it properly generate "[Accepted]", "[Resigned]", etc.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11114, T10978

Differential Revision: https://secure.phabricator.com/D17191