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 implementations of all these methods are just copy/pasted boilerplate anyway (lots of $this->subject = $subject;) and this will let Adapters support other message media (SMS, APNS, Whatsapp, etc.)

That's a larger change, but move toward a world where we can build a concrete $message object for "email" or "sms".

The PhabricatorMailEmailMessage object is just a dumb, flat object representation of the information an adapter needs to actually send mail. The existing PhabricatorMetaMTAMail is a much more complex object and has a lot of rich data (delivery status, related object PHIDs, etc) and is a storage object.

The new flow will be something like:

• PhabricatorMetaMTAMail (possibly renamed) is the storage object for any outbound message on this channel. It tracks message content, acceptable delivery media (SMS vs email), delivery status, related objects, has a PHID, and has a daemon worker associated with delivering it.
• It builds a PhabricatorMailExternalMessage, which is a simple, flat description of the message it wants to send. The subclass of this object depends on the message medium. For email, this will be an EmailMessage. This is just a "bag of strings" sort of object, with appropriate flattened values for the adapter to work with (e.g., Email has email addresses, SMS has phone numbers).
• It passes the ExternalMessage (which is a MailMessage or SMSMessage or whatever) to the adapter.
• The adapter reads the nice flat properties off it and turns them into an API request, SMTP call, etc.

This is sort of how things work today anyway. The major change is that I want to hand off a "bag of strings" object instead of calling setX(), setY(), setZ() with each individual value.

Test Plan: Grepped for MetaMTAAttachment. This doesn't change any behavior yet.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T9141

Differential Revision: https://secure.phabricator.com/D19954