Server-side validation of Battleship placements

[shtwzrd]

When the client has finished placing their ships onto the board, the configuration of ships should be POSTed to the server (via /session//placement or similar), and the server should return HTTP Status Code 200 if it's valid or 400 if invalid.

If the configuration is valid, the server's gamestate should be updated accordingly.

• Server-side domain logic for how ships can be placed #19