Suppress join/leave notifications

[Ross Boucher]

I'd love a way to disable these notifications. On a busy channel, conversations become difficult to follow (especially because the notice takes up a lot of vertical space).

[Axel Havukangas]

I suppose it could be made configureable in the channel and / or profile settings. Not a priority for me at the moment, since there are still work to be done on more general usability issues.

I'm more than happy to take in a pull-request about this though!