Automatically schedule a reminder message when the turn changes

[cjsaylor]

https://api.slack.com/messaging/scheduling

When a player makes a move, ChessBot sends a message with the updated gameboard.

In addition to this, we can use the scheduling message API above to set a reminder in the thread to make a play after X hours (say 3 hours). If another turn is played, use the scheduling API to delete the existing scheduled message (if exists) and create another.

If the game ends, do not schedule the message.