This cuts the render time in half (from ~448ms to ~200ms on my account) per received event, as we're no longer re-mounting the entire room list and instead just the section(s) we care about.
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.
