Make the example spreadsheet scroll horizontally

This replicates how we let large spreadsheets scroll horizontally.

Pro: this looks nicer and is more usable

Con: the code for this feels a bit fragile, especially the calling of
`.maintainWidth` twice, ie as many times as a it takes to get stuff to
render properly.

app/assets/javascripts/fullscreenTable.js

@@ -30,6 +30,8 @@
         window.GOVUK.stopScrollingAtFooter.updateFooterTop();
       }
 
+      this.maintainWidth();
+
     };
 
     this.insertShims = () => {