Swiss tournaments work, with basic functionality.

 - The major limitation now is that the system won't generate pairings
   if it needs to pair two players who have played each other.

 - This should never be a problem in tournaments where #players >>
   #rounds, but can be problematic in small tournaments where players
   leave.

 - Tournament doesn't auto-finish if no pairings are possible. Expect
   confused players.

 - All players need to be back to the lobby for pairings to trigger.
   Subject to exploitation by bad faith actors.