Notify players when a game has a custom solution

[JoeBussard]

Need to include a field in JSON similar to

{"is_custom_solution": true}

so that users know ahead of time

[JoeBussard]

What I'll actually do is add a variable called is_custom_game to the GameState class instance.

Then I'll have to add that variable to the list of vars published in the JSON response in backend_run_game.prepare_json_response