Change method of getting scrollTop

The current method reports `0` in Safari, whatever
the scroll position.

Some testing across our browser matrix shows:
1. this is also the case on IOS
2. browsers in the matrix all support
   `$(window).scrollTop()`

https://scroll-position-test-page.glitch.me/#results

This changes the method to use
`$(window).scrollTop()` instead.

app/assets/javascripts/fullscreenTable.js

@@ -65,7 +65,7 @@
     this.maintainHeight = () => {
 
       let height = Math.min(
-        $(window).height() - this.topOffset + $('html, body').scrollTop(),
+        $(window).height() - this.topOffset + $(window).scrollTop(),
         this.nativeHeight
       );