Propagate unread status changes to threads in update_metadata.

Test Plan:
Mark a Gmail thread as read in the web interface, and check that the
change gets propagated to the API response for the thread, when it wouldn't
before.

Reviewers: spang

Reviewed By: spang

Differential Revision: https://review.inboxapp.com/D359
Don't interpolate folder names into non-unicode strings.

This causes a crash when folder names contain unicode.