Allow creating games with different world sizes #800
Description
Right now we always use a standard size world, but we have the buttons for the other sizes, they're just hidden.
We hardcoded the number of opponents in the player setup logic, so that would need to change.
This issue will require loading some new information like the world sizes in ImportCiv3.cs and storing it in our JSON format, so this is a bit more of an involved issue, but possibly a good first issue if you're interested in some of the data format stuff.