fix/breadcrumbs-icon-hide-screenreader (#318)

* Updated breadcrumbs to hide the right-arrow icon from screen readers

* Reinstating pseudo css and removing the material icon and adding "alternative text" to the pseudo element

src/components/breadcrumbs/_breadcrumbs.scss

@@ -13,7 +13,7 @@
 
     &::before {
       @include material-icons;
-      content: 'keyboard_arrow_right';
+      content: 'keyboard_arrow_right' / '';
       font-size: rem(map-get($nsw-icon-sizes, 20));
       line-height: rem(8px);
       display: inline-block;