When setting the poster image for a video shortcode, set that image as the featured image for that attachment (if found) in the background. This AJAX functionality could be used for audio as well.

Introduces `attachment_url_to_postid()` to attempt to turn URLs into post IDs.

Fixes #27891.

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git-svn-id: http://core.svn.wordpress.org/trunk@28817 1a063a9b-81f0-0310-95a4-ce76da25c4cd