Account for `preload="metadata"` in audio and video shortcodes when multiple versions of the same `<source>` appear on the page by appending a query arg to "cache-bust" passed sources. The query arg is used to make the sources "unique." They will still be cached by the browser and won't bust the cache on every request.

Fixes #26779.



git-svn-id: https://develop.svn.wordpress.org/trunk@27519 602fd350-edb4-49c9-b593-d223f7449a82