Allow per-room notification settings/muting

[nesium]

It would be great to be able to mute notifications for high-traffic rooms, while still receiving notifications for messages in e.g. Direct Messages.

Slack offers to either mute a channel or to change the notification settings. The latter with the option to e.g. still be notified when someone mentions you.

The settings should most likely be synchronized between clients. If it's not too much work we could do this in two steps:

1. Save settings locally
2. Migrate settings to core to sync them between clients

In any event we'll have to agree on the exact set of features first.