[drafts/attachments] More proper management of attachments when saving drafts.

Summary:
Apparently we had two bugs in how we were managing attachments on drafts.
1. We were unable to remove attachments in any way
2. Each time we saved the draft we could create a new 'Part' object for each block, thus
if you save a draft several times in a row, it would create several copies of those attachments.

Test Plan: regression

Reviewers: emfree

Reviewed By: emfree

Differential Revision: https://review.inboxapp.com/D488