On server, allow setting up fixtures in advance
For tournament support.
- Depends on #17 being completed
- Admins can set up a fixture on the server, with a game config and specific players.
- When the fixture is set to "visible", players will be able to join the game, but only if they're listed as allowed players.
- A fixture can be associated with a tournament, in which case the played match will also be associated with that tournament.
- (Maybe) allow teams to be set up within a tournament, and allow adding the team to the fixture so that you don't have to manually add every player.