Summary:
This diff replaces "finalizeSessionBeforeSending" with a
plugin hook that is bidirectional and allows us to put the draft in
the "ready to send" state every time we save it, and restore it to
the "ready to edit" state every time a draft session is created to
edit it.
This diff also significantly restructures the draft tasks:
1. SyncbackDraftUploadsTask:
- ensures that `uploads` are converted to `files` and that any
existing files on the draft are part of the correct account.
1. SyncbackDraftTask:
- saves the draft, nothing else.
3. SendDraftTask
- sends the draft, nothing else.
- deletes the entire uploads directory for the draft
Test Plan: WIP
Reviewers: juan, evan
Reviewed By: evan
Differential Revision: https://phab.nylas.com/D2753
