Normally the "n" key skips over muted streams, but if we
are currently narrowed inside a muted stream, it will now
go to the next topics within that stream.
For me the use case was that I have a stream I check up on
about once a day, and "n" would be super useful for me to
clear out unread counts while still skimming some content,
and without having to temporarily unmute the stream.