I thought it would be nice to factorize `tags` and `categories` as they could be seen as specializations of `classifications`.
As the others (including `tags`) are now grouped in the `classifications` hash why not add `categories` too ?

Having the same behavior would also make it easier to merge `populate_categories` code with `populate_classifications` and avoid code duplication as the only difference between `categories` and other `classifications` is that they are downcased to generate nice folders & urls.