Add a message to flags created because of the newuser_spam_host_threshold site setting

app/models/user.rb

@@ -456,7 +456,7 @@ class User < ActiveRecord::Base
     admin = Discourse.system_user
     topic_links.includes(:post).each do |tl|
       begin
-        PostAction.act(admin, tl.post, PostActionType.types[:spam])
+        PostAction.act(admin, tl.post, PostActionType.types[:spam], message: I18n.t('flag_reason.spam_hosts'))
       rescue PostAction::AlreadyActed
         # If the user has already acted, just ignore it
       end

config/locales/server.en.yml

@@ -1162,3 +1162,5 @@ en:
 
   flag_reason:
     sockpuppet: "A new user created a topic, and another new user at the same IP address replied. See the flag_sockpuppets site setting."
+    spam_hosts: "This user tried to create multiple posts with links to the same domain. See the newuser_spam_host_threshold site setting."
+