Summary:
When you have your "Download attachments for new mail" setting set
to "manually", inline images always appear broken with no explanation.
This patch listens for the image load to fail and displays a button which
queues the fetchFile task on click. This seemed like the best approach because
it doesn't slow down the loading of the message with more fstats / lookups.
(Seeing if the file has already been downloaded is an async operation)
Test Plan: No specs atm
Reviewers: evan, juan
Reviewed By: juan
Differential Revision: https://phab.nylas.com/D3313
