dmails: send automated dmails from Danbooru.config.system_user.

Sends automated dmails from `Danbooru.config.system_user`, rather than
whichever user is performing the action happens to be (usually User.admins.first).

Also adds a notice in the view that the dmail was automated.

app/logical/approver_pruner.rb

@@ -22,7 +22,7 @@ class ApproverPruner
           user.save
         end
  
-        Dmail.create_split(
+        Dmail.create_automated(
           :to_id => user.id,
           :title => "Approver inactivity",
           :body => "You haven't approved a post in the past three months. In order to make sure the list of active approvers is up-to-date, you have lost your approver privileges. Please reply to this message if you want to be reinstated."

app/logical/user_promotion.rb

@@ -84,7 +84,7 @@ private
   end
 
   def create_dmail
-    Dmail.create_split(
+    Dmail.create_automated(
       :to_id => user.id,
       :title => "You have been promoted",
       :body => build_messages