Customize: Use `esc_url_raw()` instead of `wp_json_encode()` to eliminate...

Customize: Use `esc_url_raw()` instead of `wp_json_encode()` to eliminate extraneous slashes when outputting background image URL in CSS `url()`.

Props tyxla, westonruter.
See #22058.
Fixes #39145.

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git-svn-id: https://core.svn.wordpress.org/trunk@39486 1a063a9b-81f0-0310-95a4-ce76da25c4cd