Summary:
- You cannot drag an attachmentComponent unless it represents a fully downloaded file on disk.
- You cannot drag and drop an item from a composer onto the same composer.
- Attachments on expanded messages start downloading as soon as it's viewed (necessary for drag and dropping attachments, also probably best. Previously was just images.)
- You can now cancel an attachment download. Was really broken.
Test Plan: Run many new specs!
Reviewers: evan
Reviewed By: evan
Maniphest Tasks: T1196
Differential Revision: https://phab.nylas.com/D1764
