Now that we have more subgroups, it was an issue that visiting a parent
group would always show the topics from all of the subgroups, regardless
of whether you were subscribed to them or not.

This changes it so that, by default, only topics from subgroups the user
is subscribed to will be shown. There is also a link at the top of the
listing to toggle to the other view (all subgroups or only subscribed
subgroups).