Rewrite how the chessboard works.

Rewrite how the chessboard works.

Lots of changes; the board is now completely responsive, with everything
being positioned in terms of 12.5% increments relative to the board
(and the grid being an actual CSS grid). Animated pieces are now simply
being moved, instead of making a separate piece for the animation and
deleting it later.

It's not much smaller and I'm not sure if it's actually faster,
but it should make the problems of phantom pieces somewhat smaller.
It should certainly fix the off-grid pieces showing up.