Support automatic user creation via NID authentication by default

[jeremyfelt]

When a visitor accesses a WSU site's wp-login.php and authenticates with a valid WSU NID, they should be added as a global WSUWP user and as a user of a certain role on that site.

It's possible that Subscriber is not the appropriate role for new users. We may want to create a more restricted custom role or add the user with no role assigned.

This should be tested heavily with restricted sites. It should not be possible for a user to authenticate on a restricted site and access content.

Work for this will likely be done through the WSUWP SSO Authentication plugin, though another more specific registration plugin may be useful as well.