Add placeholder thumbnail on detail page

nova/templates/dataset/detail.html

@@ -8,14 +8,17 @@
   </div>
 </div>
 <div class="row">
-  <div class="col-md-4">
+  <div class="col-md-3">
+    <img class="img-responsive" src="https://placeholdit.imgix.net/~text?txtsize=33&txt=%C3%97&w=128&h=128"/>
+  </div>
+  <div class="col-md-3">
     {% if dataset.type == "samplescan" %}
     <p>Genus: <a href="/search?genus={{ dataset.genus.id }}">{{ dataset.genus.name}}</a></p>
   </div>
-  <div class="col-md-4">
+  <div class="col-md-3">
     <p>Family: <a href="/search?family={{ dataset.family.id }}">{{ dataset.family.name}}</a></p>
   </div>
-  <div class="col-md-4">
+  <div class="col-md-3">
     <p>Order: <a href="/search?order={{ dataset.order.id }}">{{ dataset.order.name}}</a></p>
     {% endif %}
   </div>