Allow users to override default gallery settings in JS via `wp_localize_script()`. This is already possible in PHP by filtering `shortcode_atts_gallery`.

Fixes #28693.

Built from https://develop.svn.wordpress.org/trunk@28990


git-svn-id: http://core.svn.wordpress.org/trunk@28778 1a063a9b-81f0-0310-95a4-ce76da25c4cd