Added working related contents list

Also separated related contents into partials.

app/views/relationable/_related_content.html.erb

@@ -0,0 +1,15 @@
+<div class="related-content padding">
+
+  <div class="margin-bottom">
+    <h2 class="inline-block">
+      <%= t("related_content.title") %>&nbsp;<span>(<%= relationable.relationed_contents.count %>)</span>
+    </h2>
+
+    <a data-toggle="related_content" class="add-related-content">
+      <%= t("related_content.add") %>
+    </a>
+  </div>
+
+  <%= render 'relationable/related_list', relationable: relationable %>
+
+</div>