User can join multiple rooms

[zpChris]

To the best of my knowledge, nothing is preventing the same user from joining multiple rooms simultaneously other than the inherent setup we have where a new user is created on any given "join room". Since I think it would make sense to maintain the invariant that a user can only be in one room at any time, we may want to fix this bug by updating the joinRoom and other endpoints by confirming that the user is not already in another room.