Public registration or by invitation only

The site parameter for public registration was previously a Boolean. 

This has been changed to a choice list, with the following options available: 

• Not authorized: no external account can be created
• By invitation only: a user may register and access restricted-access pages on the site if invited by a site administrator.
• Public: users can register freely to consult the site's restricted-access pages. 

Note: A script migration tool allows you to migrate existing sites: if the "Public registration" box was checked, the parameter becomes "public", otherwise it becomes "unauthorized".