feat: support for configuring the default spam threshold values for new servers

config/postal.defaults.yml

@@ -15,6 +15,8 @@ general:
   maximum_hold_expiry_days: 7
   suppression_list_removal_delay: 30
   use_local_ns_for_domains: false
+  default_spam_threshold: 5.0
+  default_spam_failure_threshold: 20.0
 
 web_server:
   bind_address: 127.0.0.1