R1:73e3057c52f4

Rename "MetaMTA" mail attachments and add more mail message objects

Summary:
Depends on D19953. Ref T9141. We have a "MetaMTAAttachment" object, rename it to "MailAttachment".

Also add a "Header" object and an "EmailMessage" object. Currently, mail adapters have a large number of methods like `setSubject()`, `addTo()`, etc, that I would like to remove.

I'd like the API to be more like `sendMessage(PhabricatorMailExternalMessage $message)`. This is likely a significant simplification anyway, since the…