Improve face animations

Use two elements, front and back; change the face variant in use
by manipulating a data- attribute on the face element; modify CSS
so it uses a selector based on that attribute.

This gives better control than the previous approach, which
updated a single element, based on properties of the container
(the hangman div itself). We can now hide the back face, then
manipulate its classes invisibly and show it when we're ready.