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.
(all the calls, feel free to switch to the new one, but in case an external plugin has been using the function, I'd like to alias the old one to the new one or vice versa, so we don't break things)