Add blog frontpage shortcode (#488)

Author: jarrodmillman

layouts/shortcodes/blog.html

@@ -0,0 +1,11 @@
+<section class="content-padding flex-row">
+  <div class="content-container">
+    {{ $latest := (first 8 (where $.Site.RegularPages.ByPublishDate.Reverse "Section" "posts")) }}
+
+    {{ partial "posts/list-with-summary.html" $latest }}
+
+    <div class="panel-box-action" style="text-align: left;">
+      <a class="panel-button" href="{{ "posts/" | relURL }}">Browse all posts &nbsp;⟶</a>
+    </div>
+  </div>
+</section>