1. There is no index on system_note_metadata's action column, so add a partial
   one on 'weight'.
2. update_column_in_batches expects a single column to be updated, but we need
   to null out note_html too. Using an SQL literal lets us get around this.
3. Arel's UpdateManager doesn't support joins, so use a subquery instead.
4. MySQL can't update from the same table referenced in the subquery. We use an
   alias to handle this case.