We previously had 2 mechanisms for narrowing used by the left sidebar
-- the top few links used the hashchange mechanism, while the streams
links used a custom click handler. Both were buggy -- the hashchange
one hadn't been updated to just select the first unread message,
whereas the click handler didn't change tabs.
Fixes#1141.
(imported from commit 8a8af974e78cc5c33937ac0078f04a9b5452b94a)