[SCHEMA] Sending mail

Summary: This is the big sending mail project which Kavya and MG have been working on. It includes basic support for composing messages, specifying recipients, and sending with the Gmail SMTP provider. It also handles the reconciliation of immediately-saved sent messages from Inbox and later sync'd messages in `Sent Mail` through a custom header.

Test Plan: Testing database should be updated. Also see `tests/imap/test_sendmail.py`

Reviewers: spang, emfree

CC: kav-ya

Differential Revision: https://review.inboxapp.com/D104