This commit doesn't actually add the final UI to zoom/unzoom
topics, because I want to keep those in separate commits, in
case we change how to enable the feature. But this commit
adds a toggle_zoom() function that zooms/unzooms topics.
Zooming is minimally invasive, because we don't really introduce
many extra elements to the UI; instead, we just make the list of
streams be a list of length one (i.e. the active stream). This
gives us a lot of stuff for free, basically, like unread counts, etc.
(imported from commit 814c1361b6210d1591b4174bed1d6e0c98a3f255)
44f575c812