Detect duplicate accounts after login instead of before registration

As we prepare to migrate to KeyCloak, #3338978: Replace Bakery with KeyCloak SSO & social sign on, we must replace all functionality customizing the login & account creation process. While KeyCloak is customizable, we are better able to maintain Drupal customizations. So we are moving the functionality within Drupal.

Currently, we block account creation when we can detect that browser already has an account on Drupal.org. We are moving this to after account creation, once someone lands back on www.drupal.org.

These merge requests make this change:
https://git.drupalcode.org/project/drupalorg/-/merge_requests/142
https://git.drupalcode.org/project/bluecheese/-/merge_requests/24