Email details body: Replace "Organizer" with "Contact"

"Organizer" isn't always accurrate if the organizer isn't actually
organizing the event but maybe only selling the tickets

src/pretix/base/templates/pretixbase/email/order_details.html

@@ -33,7 +33,7 @@
         </tr>
         <tr>
             <td>
-                <strong>{% trans "Organizer:" %}</strong>
+                <strong>{% trans "Contact:" %}</strong>
             </td>
             <td>
                 {{ event.organizer }}
@@ -121,7 +121,7 @@
         {% endif %}
         <tr>
             <td>
-                <strong>{% trans "Organizer:" %}</strong>
+                <strong>{% trans "Contact:" %}</strong>
             </td>
             <td>
                 {{ event.organizer }}