[SCHEMA] Fixes T163: Drafts synced from remote can't be updated/sent via API

Summary:
Kill SpoolMessage, the Message table stores the extra info needed for Inbox-created, synced back drafts.

Also fixes a few minor drafts related bugs.

Test Plan: None (regression tested)

Reviewers: emfree

Reviewed By: emfree

Differential Revision: https://review.inboxapp.com/D255