Allow modular transactions to override transaction title and body text in mail

Summary:
Ref T12921. I'm moving Instances to modular transactions, and we have an "Alert" transaction type used to send notifications ("Your instance is going to be suspended for nonpayment.").

Currently, there's no way to specifically customize mail rendering under modular transactions. Add crude support for it.

Note that (per comment) this is fairly aspirational right now, since we actually always render everything as text (see T12921). But this API should (?) mostly survive intact when I fix this properly, and allows Instances to move to modular transactions so I can fix some more pressing issues in the meantime.

Test Plan: See next diff for Instances.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T12921

Differential Revision: https://secure.phabricator.com/D20057