Rename jetpack_get_youtube_id to jetpack_shortcode_get_youtube_id

https://github.com/Automattic/jetpack/blob/master/class.media-extractor.php#L161 is expecting all "special" shortcodes to have a get ID function in the format of `jetpack_shortcode_get_{shortcode}_id`.

91a3bdd3 named the function `jetpack_get_youtube_id`. Since the extractor couldn't get the ID, this created Automattic/Jetpack#930 since the media summary didn't have the expected ID ( https://github.com/Automattic/jetpack/blob/master/class.media-summary.php#L69 ).

Renaming all instances to jetpack_shortcode_get_youtube_id resolves this. Sure there are other ways too. 

Fixes Automattic/jetpack#930.