Fix the tall menu problem

- if the height of the contents list exceeds the height of the viewport users can't see the whole contents unless they scroll to the bottom of the page, where the sticky element hits the bottom and stops
- to fix this, we apply a max height of the viewport and an overflow y scroll, so that the element never exceeds the height of the viewport and can be scrolled
- also adding a bottom margin to prevent collision with the back to top link at the bottom

Author: andysellick

app/assets/stylesheets/views/_html-publication.scss

@@ -25,6 +25,9 @@
     @include govuk-media-query($from: desktop) {
       position: sticky;
       top: govuk-spacing(2);
+      max-height: 100vh;
+      overflow-y: auto;
+      margin-bottom: govuk-spacing(4);
     }
 
     .direction-rtl & {