Summary:
We used to use a progress-based spinner, but we never get any progress
updates while downloading files, so the spinner was always at 0 and
never visible.
The submodule-commit is D3609, which adds support for inline images
Test Plan: tested locally
Reviewers: evan, spang
Reviewed By: spang
Differential Revision: https://phab.nylas.com/D3610
