Customizer: Add support for the media library in Background Image controls.

`WP_Customize_Upload_Control` (PHP) and `api.UploadControl` (JS) can now open the media modal.

props celloexpressions, and ericlewis for docs.
see #21483.
Built from https://develop.svn.wordpress.org/trunk@30309


git-svn-id: http://core.svn.wordpress.org/trunk@30308 1a063a9b-81f0-0310-95a4-ce76da25c4cd