Summary:
Adds a UI component to see the progress of initial sync.
(The behavior is sometimes a little strange, but I believe this is
just because of weird behavior in the NylasSyncStatusStore itself.
For instance, it sometimes never populates the `folderSyncProgress`
fields, or it never sets the progress to 1, so the bar stops at 99%)
Test Plan: tested locally
Reviewers: juan, evan
Reviewed By: evan
Differential Revision: https://phab.nylas.com/D3536
