Add content_id to API so clients can support embedded images

Summary:
The HTML body of a message can contain image tags which reference a `cid` aka Content-ID. This is a string that represents which MIME attachment to display inline. Since we're not modifying the HTML, we need to expose the Content-ID value so clients can (optionally) load and render the image inline via our API.

More info on Content-ID: https://www.ietf.org/rfc/rfc2392.txt

Test Plan: passes tests

Reviewers: mg, spang

Reviewed By: spang

Subscribers: kav-ya, emfree

Differential Revision: https://review.inboxapp.com/D756