Add small timer before returning to the board and require the field to be fully matched in memory minigame

To prevent players blocking each other from completing the game, when holding a card open for more than 5 seconds, it will be automatically flipped down (and a small penalty applied, so you cannot instantly pick it up) so the other player on your half of the board has a chance to pick it up.

Doing just a timer as in #199 (closed) looked a bit weird, with an unmatched board

Resolves #199 (closed)