Return more attachment information in API message representation.

Test Plan: curl http://localhost/n/<ns_id>/messages/<id of message with attachment>

Reviewers: charles

Subscribers: mg, bengotow

Differential Revision: https://review.inboxapp.com/D268
Schedule draft-deletion-syncback as a separate action from sending.

This prevents us from potentially re-sending a message if the drafts-syncback
call fails for some separate reason.