Add missing Outlook conditional comment

Adds a missing closing tag for an Outlook conditional comment. We had an
`if !mso` conditional comment targeting non-Outlook clients. By failing
to close this, we accidentally marked all subsequent HTML in the email
as being for non-Outlook clients only. This meant that in classic
Outlook, all of this content was ignored and not displayed, including
all of the user data in the email.

Author: DavidBiddle

app/views/layouts/mailer.html.erb

@@ -117,6 +117,7 @@
                         </table>
                       </a>
                     </td>
+                    <!--<![endif]-->
                   </tr>
                 </table>
                 <!--[if (gte mso 9)|(IE)]>