FIX: PM should never be allowed to have a category

FIX: TL3 should not be allowed to muck with PM titles

db/migrate/20140911065449_private_messages_have_no_category_id.rb

@@ -0,0 +1,9 @@
+class PrivateMessageShaveNoCategoryId < ActiveRecord::Migration
+  def up
+    execute "UPDATE topics SET category_id = NULL WHERE category_id IS NOT NULL AND archetype = \'private_message\'"
+    execute "ALTER TABLE topics ADD CONSTRAINT pm_has_no_category CHECK (category_id IS NULL OR archetype <> 'private_message')"
+  end
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end