fixes IE8 issue with "+/-" icons not being toggled

For IE8 css pseudo elements, we needed to force a repaint to toggle
the icons. This is done by using CSS content property.

https://stackoverflow.com/questions/8703799/forcing-ie8-to-rerender-repaint-before-after-pseudo-elements

src/components/accordion/_accordion.scss

@@ -166,6 +166,7 @@
 
     // Vertical bar should be hidden when section is open, to display a '-' icon
     .govuk-accordion__section--expanded .govuk-accordion__icon:after {
+      content: " ";
       display: none;
     }
   }