Previous code iteratively called set_user_status() once for each user;
which in turn was rerendering the user sidebar, also once for each user.
I changed the API a bit by replacing activity.set_user_status() with
activity.set_user_statuses(), plural, that takes an object and updates
all the user statuses in one go before rendering.
(imported from commit 1111c9029264f892f25e76d2e5e5ff996dcbc7ca)