mailman3: Disable Gravatar integration

We don't want our users tracked by a third party.

roles/mailman3/templates/settings.py.j2

@@ -47,6 +47,7 @@ SERVER_EMAIL = 'root@{{ lists_domain }}'
 
 POSTORIUS_TEMPLATE_BASE_URL = 'http://localhost'
 HYPERKITTY_ALLOW_WEB_POSTING = False
+HYPERKITTY_ENABLE_GRAVATAR = False
 
 HAYSTACK_CONNECTIONS = {
     'default': {