When a plain URL or email address is selected in the editor and "Insert/edit link" is clicked, pre-populate the URL field with the URL or `mailto:<email-address>`.

Props tillkruess.
Fixes #19992.


git-svn-id: https://develop.svn.wordpress.org/trunk@28705 602fd350-edb4-49c9-b593-d223f7449a82