Add mail support to generic transactions

Summary:

• Adds mail support to the generic transaction construct.
• Restores mail support to Pholio (now much improved; the mails are actually useful).

Test Plan: Updated a Pholio mock, got mail.

Reviewers: btrahan, chad, vrana

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2104

Differential Revision: https://secure.phabricator.com/D4139