When user is mentioned (via matrix 'highlight_count'), inactive
rooms will use a different color for the notification circle than
when only general unread messages exist.
The old algorithm during filtering will check every room
if it's part of the group. O(N*G)
The room ids for a group are now stored in a map for faster lookup
so the search can be completed in a single pass. O(N)
Emi Simpson
Nicolas Werner
redsky17
Konstantinos Sideris