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/dmail.rb

@@ -45,20 +45,24 @@ class Dmail < ActiveRecord::Base
         copy = nil
 
         Dmail.transaction do
+          # recipient's copy
           copy = Dmail.new(params)
           copy.owner_id = copy.to_id
-          unless copy.to_id == CurrentUser.id
-            copy.save
-          end
+          copy.save unless copy.to_id == copy.from_id
 
+          # sender's copy
           copy = Dmail.new(params)
-          copy.owner_id = CurrentUser.id
+          copy.owner_id = copy.from_id
           copy.is_read = true
           copy.save
         end
 
         copy
       end
+
+      def create_automated(params)
+        create_split(from: Danbooru.config.system_user, **params)
+      end
     end
 
     def build_response(options = {})
@@ -186,6 +190,10 @@ class Dmail < ActiveRecord::Base
     end
   end
 
+  def is_automated?
+    from == Danbooru.config.system_user
+  end
+
   def filtered?
     CurrentUser.dmail_filter.try(:filtered?, self)
   end