Support for attachments in drafts

Summary: This adds support for attachments in drafts and sent messages. We pass around block_public_ids (externally known as file ids) until we create the MIME representation of the message. The function `gen_attachments` in inbox/server/sendmail/base.py converts a list of Block public_ids to a dicionary containing attachment metadata and the actual data as a bytestring.

Test Plan: Tests have been modified and pass

Reviewers: kav-ya, emfree

Reviewed By: emfree

CC: emfree

Differential Revision: https://review.inboxapp.com/D176