public static object GetAcceptInvitationForm(
            NancyContext context,
            IDocumentSession documentSession,
            string invitationCode)
        {
            context.Ensure("context");
            documentSession.Ensure("documentSession");

            if (!context.IsSignedUp())
                return 403;

            var playerInvitation = documentSession.GetPlayerInvitationByCode(invitationCode);
            if (playerInvitation == null || playerInvitation.Used)
                return 404;

            var room = documentSession.GetRoomById(playerInvitation.RoomId);
            if (room == null)
                return 404;

            return new AcceptInvitationFormResponse(room, invitationCode);
        }