Correspondence Chess

As you may noticed already, correspondence chess is available now on the PyChess site. First I thought it will be casual only, but after @CouchTomato87 and @SteveatKurihara noticed rated correspondence games are available on Lishogi and on all correspondence chess sites. So I changed my mind and implemented rated play also. I know Lichess doesn't allow it, because there were lots of complaint about how it has affects the leaderboards. We will see. For now, the ratings are not separated from real time variant ratings, similar to Lishogi.

The rules are the same as on lichess/lishogi. Using everything is allowed except using engines!

Implementing this feature affected almost all of the code base, so it's very possible it will bring new bugs and some regressions. Please feel free to report bugs on GitHub or Discord

-gbtami