Muting a channel should also disable role-based notifications in that channel

For instance if I mute the #roguelikes-livestream channel and then someone pings @angband in that channel, I should not receive a notification for that ping, even if I belong to the associated role to receive pings from @angband. I might want to receive @angband pings in other channels, but I might not be interested in live streams.

This can be particularly annoying if I have the channel hidden as well, as then I have to hunt all over to find out where this notification is coming from.