Drupal 9 port

Looks like https://www.drupal.org/project/content_roles → may be doing a very similar thing. If so, perhaps we should recommend am upgrade path to that module for D9?

Hmm...it's not quite the same, and I did want a D9 port of this module, so I've done it anyway.

Automatically closed - issue fixed for 2 weeks with no activity.