Add parallax effect to background

[Wirewraith]

The old site had a parallax effect for the debris and floating spaceman background overlays. It was fairly neat so we might as well copy it.

[AffectedArc07]

LetterN got parallax working for a react rewrite of the webmap.

I know this runs on vue but a react component may provide pointers, https://github.com/LetterN/SS13WebMap/blob/neutrinos/reactive-station/src/ParallaxBG.tsx