FEATURE: automatically archive welcome messages for site_contact_user

This de-clutters the sent messages box for site_contact_user, making it again usable

db/migrate/20160110053003_archive_system_messages_with_no_replies.rb

@@ -0,0 +1,24 @@
+class ArchiveSystemMessagesWithNoReplies < ActiveRecord::Migration
+  def up
+    # backdate archival of system messages send on behalf of site_contact_user
+    execute <<SQL
+
+    INSERT INTO user_archived_messages (user_id, topic_id, created_at, updated_at)
+    SELECT p.user_id, p.topic_id, p.created_at, p.updated_at
+    FROM posts p
+    JOIN topics t ON t.id = p.topic_id
+    LEFT JOIN user_archived_messages um ON um.user_id = p.user_id AND um.topic_id = p.topic_id
+    WHERE t.subtype = 'system_message' AND
+          t.posts_count = 1 AND
+          t.archetype = 'private_message' AND
+          um.id IS NULL AND
+          p.user_id IS NOT NULL AND
+          p.topic_id IS NOT NULL AND
+          p.post_number = 1
+SQL
+
+  end
+
+  def down
+  end
+end