reconnection works now if the rejoining player enters the passphrase
again.
Some bugs are still happening:
1. The rejoining client is not told the state of the board
2. Invalid moves are not handled by the client (not sure why though)
3. The still-connected client should be told, that the opponent
disconnected. Then the client should show the passphrase again
With this commit, we stop waiting for the websocket connection to be
established before the game starts.
Now, the Connection type is responsible for waiting for the websocket
connection before writing.
In order to simplify special moves like en passant or castling for the
client, we want to deliver the board state after every move (and not
only start square and end square).
With PGN we can encode a chess position into a string.
This commit implies changes to logic of the pieces' shortnames. This
will break the client/server connection (at least for promotions).
