Add subtype to topics to classify private messages

spec/components/system_message_spec.rb

@@ -1,5 +1,6 @@
 require 'spec_helper'
 require 'system_message'
+require 'topic_subtype'
 
 describe SystemMessage do
 
@@ -20,6 +21,10 @@ describe SystemMessage do
       topic.should be_private_message
     end
 
+    it 'should have the correct topic subtype' do
+      topic.subtype.should == TopicSubtype.system_message
+    end
+
     it 'should be visible by the user' do
       topic.allowed_users.include?(user).should be_true
     end