This avoids a stream having potentially near-infinite height when
opened in a stream with a large number of unread topics; the benefit
is that you can easily access the next stream.
We show an unread count next to "more topics" to make it hard to miss
that there might be more, older topics with unread messages.
With CSS work by Anders Kaseorg.
Fixes#13087.
Signed-off-by: Anders Kaseorg <anders@zulipchat.com>