This used to be a feature but kinda broke/changed when we switched to
focusing on context-aware or notable events. Basically instead of only
notable events making a room bold in the room list, it's any unread
messages now.
This is only called after we already get an event with lastEvent() so
doing it again is useless. Instead, we should refactor it to behave like
similar functions (e.g. isEventHighlighted.)
