Players currently can join multiple lobbies at the same time

This may be achieved by the following:

1. Open the game in a browser tab
2. Duplicate the tab
3. Join a lobby at Tab 1
4. Join another lobby at Tab 2

Not sure if this is intended, but from what I expect/understand, usually multiplayer games enforce some sort of "1 lobby per player" rule.

Reproduce the behavior in a browser by duplicating the game tab and joining different lobbies from each tab. Trace the lobby-join handling and confirm the intended one-lobby rule with the issue discussion; done means the behavior matches the agreed rule and the two-tab scenario is covered by a test.

Written by the indexing model from the issue text.