Fixes https://github.com/vector-im/riot-web/issues/14694
Instead of spending 10-1000ms in a function iterating over a whole lot of room events, we can use our cached state from the Notification State Store.
This commit sets up a structure that could be applied to communities in the TagPanel too, as that could probably use a similar optimization.
This reduces the updateStatusIndicator() time to just 4ms on average.