templates: use logo as header

newsletter/templates/newsletter/email_body.html

@@ -105,7 +105,7 @@
             <table role="presentation" cellspacing="0" cellpadding="0" border="0" align="center" width="100%" style="max-width: 600px;">
               <tr>
                 <td style="padding: 20px 0; text-align: center; font-family: sans-serif;">
-                  <span style="font-size: 15px; line-height: 20px;">Cette semaine sur Radio Panik</span>
+                  <span style="font-size: 15px; line-height: 20px;"><img src="https://www.radiopanik.org/static/img/Radio_Panik_Logo_2016-01.png" alt="Radio Panik"/></span>
                 </td>
               </tr>
             </table>