Implement a system to ensure that a user cannot join multiple lobbies at once

[jiyeongchloe]

Goals: Currently, users are allowed to join any lobby they would like, including lobbies that would be playing matches concurrently. Because the match start conditions partly rely on the fact that there are enough players in the lobby to play the game, it would not make sense to let players choose which lobby/match they would like to commit to after the match is allowed to start. We need to implement a system to ensure that a user cannot join multiple lobbies at once. For example, if we detect that a user wanting to join lobby A is currently in another lobby whose match has not started yet, we can prompt the user to leave that lobby before joining lobby A.

Deliverables:

• Documentations about uniqueness constraint?
• Pull Request