Show check or X when correct, incorrect

Replace button text with unicode check mark or X mark when guess is
correct or incorrect, respectively.

This provides a second mode of feedback, the first being making the
button green or red. Will help make the app usable for people with red-
green color blindness and reinforce feedback for others.

Use heavy check mark (U+2714) but regular X mark (U+2717) so that
positive feedback has a bit more weight.