We were taking 0.2ms to handle the registration of a timer per event during startup, even before the app is visible to the user. These timers would be short-circuited too, leading to a bunch of wasted time.
0.2ms isn't a lot of time, but multiplied by thousands of events at startup it's pretty significant.
On my account this reduces the full page spinner time from ~50 seconds to just over 20 seconds.
Fixes https://github.com/vector-im/riot-web/issues/14091
Design needs work, however this is behind labs anyways. This re-implements the behaviour of the old room list.
The implementation ended up being a lot easier due to early confusion with what the TagOrderStore and TagPanel take care of. Turns out they don't deal with tags, but groups. As such, we don't need to do anything with filtering (though we keep some sanity checks in place for safety), and just have to wire up the CustomRoomTagPanel and CustomRoomTagStore.
