Add the ability to mute nicknames

This is mostly aimed at bots, which generally have a low signal-to-noise ratio.

As neat as it would be to filter out the noise, it's unlikely to happen: There's little common ground between different bots on what qualifies as signal.

So take out a biggish hammer, and allow "muting" nicks. We will still open a conversation and show messages, but we won't show a notification for them or treat them as highlighted.