Track users in the current room

[itsthesteve]

Right now the sidebar is just mock data. Depends on #1

• [ ] Update the KV store with which users are in what room
• [ ] Update the UI to show users as they come and go

Something along the lines of:

// When a room is joined, notify a table of the room and the user ID
kv.set(["room_joined", roomId, userId])

// listen for this and grab a list, maybe prefixed with `room_joined` and `room_id`...
for await (const [roomId] of db.watch([["room_joined", roomId]]) {
  ...
}

[itsthesteve]

This was completed but regressed and isn't working correctly.