wpLink: stop using UI dialog, restyle the modal, add better responsive behaviour.
Fix UI dialog styling to match the rest of the admin styling.
Props avryl, see #26952
Built from https://develop.svn.wordpress.org/trunk@27494


git-svn-id: http://core.svn.wordpress.org/trunk@27336 1a063a9b-81f0-0310-95a4-ce76da25c4cd