Add block class to to images on mobile so they behave as expected

Because we're using anchor tags, which are not block-level elements,
they images were not displaying with background color or border. Making
the elements display block fixed that. Also removed the unneeded
flex-auto from mobile.

themes/gohugoioTheme/layouts/partials/boxes-showcase-items.html

@@ -1,4 +1,4 @@
-<a href="{{.Permalink}}" class="b--moon-gray ba bg-animate bg-white flex-auto hover-bg-near-white mb3 mr3 w-100 w-40-l">
+<a href="{{.Permalink}}" class="b--moon-gray ba bg-animate bg-white db flex-auto-ns hover-bg-near-white mb3 mr3 w-100 w-40-l">
   <div class="flex flex-column black h-100 mb2 pa3 pa4-l">
     {{ $img := (.Resources.ByType "image").GetMatch "*featured*" }}
     {{ with $img }}