Override visibility rules for moderators

Moderators should be able to see all games since they need to be able to see if people are naming their lobbies in violation of the rules.

We should always send all games to moderators, but add a field to the ones that would normally be invisible signaling that the game is not joinable. We should then refactor the login in `command_game_join` so it actually enforces the visibility rules, since right now we are relying on the clients not knowing the game ids to prevent people from joining games that they shouldn't be able to.

I would suggest we call this new field `joinable` and make it "nullable" as in, we don't include it at all in game info messages that are sent to non-moderators. We could expand this functionality in the future to allow people to create visible, but not joinable games (not sure why we would want that, but who knows).

We will also need some client changes to differentiate between joinable and non-joinable games in the game list.