Time-based notification mute ("do not disturb")

There are often times when I'd really like to just mute notifications for all my rooms or a specific (particularly noisy) room for a few hours. In the global case, maybe you could also [set the user's presence to unavailable](https://matrix.org/docs/spec/client_server/r0.2.0.html#put-matrix-client-r0-presence-userid-status).

On the spec side, this could just be a generic expiration date for notification events, so we could do clever things by poking at the API manually. :) (Maybe even make recurring notification settings an option? E.g. "I want to get notifications for this keyword only between 9am-5pm. That would've been super useful when I was using keyword highlights for alerts at work.)

Related to https://github.com/vector-im/riot-web/issues/1672 I suppose, but I think this is more generic - I'm looking for something that allows more granular control (e.g. per-room controls).