Refactor fns that generate responses;
update implementation for promotion messages so that clients to
the game service know what position is to be promoted.
Have the 'promotion options' alert box fade in.
Modify stylesheet slightly.
Add function #generate-chessman; fix bug that causes
promotion when one's king is attacked.
Modify event system so that it can return values contingent
upon a single optional argument; add ReactJS-nodes for promotion options.