Decouple unread counters from notifications

[ghost]

Description

I need an unread count on rooms to decide which one to open next. Right now that means I have to take notification for every message in every room, because there's no way to have one without the other.

I think this is also the reason notifications are turned up so high by default in the first place.

Related

vector-im/element-web#3268

[dbkr]

Dup https://github.com/vector-im/riot-web/issues/3374