Make the CSS selector for current playlist item `<img>` more specific so the width is properly constrained.

Hide the `<audio>` element in the playlist to prevent a flicker when loading - MEjs hides the audio element anyway.

See #27525.


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git-svn-id: http://core.svn.wordpress.org/trunk@27579 1a063a9b-81f0-0310-95a4-ce76da25c4cd