R1:b5797ce60a35

Refactor mail to produce an intermediate "bag of strings" object in preparation for SMS

Summary:
Depends on D19954. Ref T920. This is a step toward a world where "Mailers" are generic and may send messages over a broader array of channels (email, SMS, postal mail).

There are a few major parts here:

- Instead of calling `$mailer->setSubject()`, `$mailer->setFrom()`, etc., build in intermediate `$message` object first, then pass that to the mailer.
- This breaks every mailer! This change on its own…