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 `$message` object first, then pass that to the mailer.
- This breaks every mailer! This change on its own does not fix them. I plan to fix them in a series of "update mailer X", "update mailer Y" followups.
- This generally makes the API easier to change in the far future, and in the near future supports mailers accepting different types of `$message` objects with the same API.
- Pull the "build an email" stuff out into a `PhabricatorMailEmailEngine`. `MetaMTAMail` is already a huge object without also doing this translation step. This is just a separation/simplification change, but also tries to fight against `MetaMTAMail` getting 5K lines of email/sms/whatsapp/postal-mail code.
- Try to rewrite the "build an email" stuff to be a bit more straightforward while making it generate objects. Prior to this change, it had this weird flow:

```lang=php
foreach ($properties as $key => $prop) {
switch ($key) {
case 'xyz':
// ...
}
}
```

This is just inherently somewhat hard to puzzle out, and it means that processing order depends on internal property order, which is quite surprising.

Test Plan: This breaks everything on its own; adapters must be updated to use the new API. See followups.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T920

Differential Revision: https://secure.phabricator.com/D19955