When rendering Underscore templates for Audio/Video details, use 2 new PHP functions to render the markup for audio and video tags: `wp_underscore_audio_template()` and `wp_underscore_video_template()`.

Future JS templates can follow the convention of expecting `data` to be passed to them containing a `model` property.

See #27389.



git-svn-id: https://develop.svn.wordpress.org/trunk@27533 602fd350-edb4-49c9-b593-d223f7449a82