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.

git-svn-id: https://develop.svn.wordpress.org/trunk@30309 602fd350-edb4-49c9-b593-d223f7449a82