Make sure the media API returns the VideoPress GUID

[dbtlr]

In creating parity of features between Jetpack and Calypso, we've run into an issue where Jetpack sites are not returning the VideoPress GUID as expected. This change adds in a stub of the similar function that WordPress.com uses in its API, so that the API endpoint will return the GUID as expected.

Note

This is currently a pretty large blocker for the VideoPress thumbnail picker in Calypso. The sooner we can get this tested and merged the better.

To test

1. Upload a Video w/ VideoPress enabled.
2. Go https://developer.wordpress.com/docs/api/console/
3. Enter the URL: /sites/$site_id/media/ w/ the jetpack site domain name as the $site_id
4. Check the response and verify that it contains the videopress_guid for any VideoPress posts that were uploaded.

[dbtlr]

Just another note, it appears that WordPress.com developer console doesn't authenticate Jetpack connections as well as I thought. The best way to test this is actually to:

1. Go to a Jetpack site in Calypso
2. Open a post
3. Open the inspector
4. Add a new media item to open the media window
5. Check the response for the /site/$site_id/media call in the network inspector for the videopress_guid

[zinigor]

I was uncertain of this because we're adding a function but not calling it anywhere, but then I realized that existing code was just wrapped in a function_exists condition. Works well, looks good!

[jeherve]

This works well in my tests. It should be good to merge after a minor change.

[jeherve]

Shouldn't we use Yoda notation here, as per the WP coding standards?

[jeherve]

Took care of it in af420e5