Customize: Prevent navigation in preview when clicking on child elements of...

Customize: Prevent navigation in preview when clicking on child elements of preview links that have non-previewable URLs.

Fixes #39098.

Built from https://develop.svn.wordpress.org/trunk@39584


git-svn-id: https://core.svn.wordpress.org/trunk@39524 1a063a9b-81f0-0310-95a4-ce76da25c4cd