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/models/user_feedback.rb

@@ -92,7 +92,7 @@ class UserFeedback < ActiveRecord::Base
   def create_dmail
     unless disable_dmail_notification
       body = %{#{creator_name} created a "#{category} record":/user_feedbacks?search[user_id]=#{user_id} for your account. #{body}}
-      Dmail.create_split(:to_id => user_id, :title => "Your user record has been updated", :body => body)
+      Dmail.create_automated(:to_id => user_id, :title => "Your user record has been updated", :body => body)
     end
   end