Fix extended footer on certain pages

Some pages (e.g. https://www.gov.uk/government/publications/armed-forces-corporate-covenant-signed-pledges) have a long revision history. This, by default, has a class of visuallyhidden that hides the extended content off screen using position:absolute and left:-9999em. While this is hidden, the viewport will extend itself to include absolutely positioned elements, even if they’re positioned horizontally off screen.

This patch clips off screen elements so that they don’t take up any vertical space. It doesn’t affect VoiceOver’s ability to read the extended content.

source/assets/stylesheets/_accessibility.scss

@@ -22,6 +22,8 @@
 .visuallyhidden {
   position: absolute;
   left: -9999em;
+  height: 0;
+  overflow: hidden;
 
   /*
    * Extends the .visuallyhidden class to allow the element to be