R1:138c07f32c79

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…