Being in streams that generate a lot of topics that have a medium chance
of being uninteresting can make it hard to keep up - muting a topic can
help, but without a key binding, it's pretty tedious.

This change adds the "M" key binding that allows muting a topic very
quickly, and an "U" key binding that allows unmuting it from the topic's
stream.

This tests that muting via the M key and un-muting via the button, as
well as un-muting via the U key in a narrowed (muted) topic work.

No longer show more than one "topic X in stream Y" is muted message
anymore.

Also, dismiss the message if the topic gets unmuted while the message is
showing (I don't expect this to be very common, but it's definitely
possible to do).

Currently if user tries to mute already muted topic, nothing shows up.
It is convenient to show a warning that the topic is already muted.