default result sorting to quiz submission descending

completes #85

app/views/admin/result/index.html.erb

@@ -8,6 +8,7 @@
       <th>Test ID</th>
       <th>Experience</th>
       <th>Recruiter</th>
+      <th>Submitted on</th>
       <th>Interview?</th>
     </tr>
 
@@ -16,6 +17,7 @@
         <td><%= link_to candidate.test_hash, admin_result_path(candidate.test_hash) %></td>
         <td><%= candidate.experience %> years</td>
         <td><%= mail_to(candidate.recruiter.email) %></td>
+        <td><%= candidate.completed_at.strftime('%D') unless candidate.completed_at.nil? %></td>
         <td><%= candidate.interview? %></td>
       </tr>
     <% end %>