Fix spacing between button and selected areas

Adding left margin to the button meant that when it wrapped onto a new
line it didn’t align flush with the left edge of its containing column.

Instead we can:
- move the link outside the list (which is better semantically anyway)
- then add the margin to the last item of the list (which is now the
  last selected area, not the link)

app/assets/stylesheets/components/area-list.scss

@@ -55,6 +55,10 @@
 
     }
 
+    &:last-child {
+      margin-right: govuk-spacing(1) * 1.5;
+    }
+
     &--unremoveable {
       padding-right: govuk-spacing(2);
       background: $light-blue-25;