crm: keep event aggregate when setting content from notification message

If event has an aggregate relation already, merge it with the
aggregate relation of notification message.

/reviewed-on nexedi/erp5!873

bt5/erp5_crm/SkinTemplateItem/portal_skins/erp5_crm/Event_setTextContentFromNotificationMessage.py

@@ -23,7 +23,8 @@ if notification_message is not None:
   mime, text_content = notification_message.convert(target_format,
       substitution_method_parameter_dict=substitution_method_parameter_dict)
   context.setTextContent(text_content)
-  context.setAggregateList(notification_message.getProperty('aggregate_list', []))
+  context.setAggregateSet(
+      context.getAggregateList() + notification_message.getProperty('aggregate_list', []))
 
   if not context.hasTitle():
     context.setTitle(notification_message.asSubjectText(