Add go-popular support to go-termtracker

See: https://github.com/GigaOM/legacy-pro/issues/4326
```php
if ( $this->show_popular  && $this->column_name( 'go_popular' ) == $column )
{
  echo $this->column_go_popular( $post_id );
}

if ( $this->column_name( 'terms' ) == $column )
{
  // escaped upstream, contains HTML
  echo echo $this->column_terms( $post_id );
}