Summary:
This is going to be a diff way shorter than the previous one! Basically, it adds a new endpoint, `/blobs` to our API to store send later attachments. When a user schedules a draft to be sent, we send all attachments to this endpoint. Separately, we store the rest of the message as metadata.
When it's time to send the message, we fetch the attachments from S3, fetch the metadata and merge them together to get a message we can send.
Test Plan: Tested manually. Will make a final QA pass before landing.
Reviewers: juan, halla, evan
Reviewed By: halla, evan
Differential Revision: https://phab.nylas.com/D4196
