rename system_username to site_contact_username , system_user is a special user with -1 id that is only used for certain admin tasks

for example system_user will autoclose stuff if needed, it will delete stubs and be the target for flag pms

lib/discourse.rb

@@ -128,12 +128,16 @@ module Discourse
     end
   end
 
-  # Either returns the system_username user or the first admin.
-  def self.system_user
-    user = User.where(username_lower: SiteSetting.system_username).first if SiteSetting.system_username.present?
+  # Either returns the site_contact_username user or the first admin.
+  def self.site_contact_user
+    user = User.where(username_lower: SiteSetting.site_contact_username).first if SiteSetting.site_contact_username.present?
     user ||= User.admins.real.order(:id).first
   end
 
+  def self.system_user
+    User.where(id: -1).first
+  end
+
   def self.store
     if SiteSetting.enable_s3_uploads?
       @s3_store_loaded ||= require 'file_store/s3_store'