Add secondary link pattern to page footer

This commit brings back the ‘link under the green button’ bit of the page footer
component.

Previous it had been changed to be a grey button. But there are use cases for
both, maybe even simultaneously.

app/assets/stylesheets/components/page-footer.scss

@@ -28,6 +28,10 @@
 
   }
 
+  &-secondary-link {
+    display: block;
+    margin-top: $gutter;
+  }
 
   .button {}