CMFActivity: Disable mail sending by default

  The administrator can enable it back clicking in a button,
  this can prevent massive email sending from unattended sites (development for example)
  related to test failures or average development

product/CMFActivity/ActivityTool.py

@@ -686,7 +686,7 @@ class ActivityTool (BaseTool):
     activity_creation_trace = False
     activity_tracking = False
     activity_timing_log = False
-    activity_failure_mail_notification = True
+    activity_failure_mail_notification = False
     cancel_and_invoke_links_hidden = False
 
     # Filter content (ZMI))