Admin Page: Domain Mapping plugins lead to CORS errors

(I thought there was an issue for this already, but search-fu failed).

When the site_url and the home_url are different, commonly with domain mapping plugins, the new admin page will often fail due to CORS errors.

For example, the wp-admin may live at http://test.example.com/wp-admin/ but the home_url (and what Core's `rest_url()` uses) is http://example.net/ , so the React requests to the REST API URLs are blocked due to CORS.

Start by reproducing the admin page with different site_url and home_url values, then inspect the React requests to the REST API URLs generated through Core's rest_url(). Done means those requests no longer fail with CORS errors when domain mapping plugins separate the wp-admin host from the home_url host.

Written by the indexing model from the issue text.