Make the images on the web page link to the JS puzzles.

I've been thinking for a while that it's about time I did that. Those
images used to link to the Java versions of the puzzles, back when
Java was the in-browser applet platform of choice; then I made them
not link to either one when it wasn't clear which system would win;
but now JS has clearly won, it's past time the images linked to the JS
puzzles, as the obviously sensible default.