Add support for more granular sending of email in application transactions

Summary: Deploy on paste and macro for create stories, 'cuz those are boring emails. Fixes T3808.

Test Plan: made a paste and a macro. commented on 'em. verified i got mail on comments only.

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, aran

Maniphest Tasks: T3808

Differential Revision: https://secure.phabricator.com/D6988