Adjust position of action link arrow

- arrow previously aligned to the top (default) whereas text is vertically aligned to the middle
- not noticeable most of the time but on the homepage, where several action links with differing text are next to each other, and an override is in place to make this change
- still not noticeable unless text size is increased (can do zoom text only in Firefox)
- therefore moving this positioning change to the component itself as the default, as it seems like a sensible option
- action link only used on homepage, browse pages

Author: andysellick

app/assets/stylesheets/govuk_publishing_components/components/_action-link.scss

@@ -12,6 +12,7 @@
   display: table-cell;
   line-height: 1;
   width: 36px;
+  vertical-align: middle;
 
   svg {
     height: 28px;