Automatically schedule a reminder message when the turn changes

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.

Start at the ChessBot move-handling path that sends the updated gameboard and review the Slack messaging scheduling API linked in the issue. The work is done when an ongoing game has one reminder replaced after each turn, while finished games do not receive a scheduled message.

Written by the indexing model from the issue text.