Set public media as public access when copying to CDN.

This will fix https://trac.silverorange.com/ticket/10967

To test, you'll need to re-queue any existing single public and single private video on a site with SiteVideoMedia, and then run the CdnUpdater and check to make sure the access controls are set correctly on the items on S3.

```sql
-- single binding for public video
select MediaEncodingBinding.media, MediaEncodingBinding.encoding from MediaEncodingBinding where media_set in (select id from MediaSet where MediaSet.private = false) limit 1;

-- single binding for private video
select MediaEncodingBinding.media, MediaEncodingBinding.encoding from MediaEncodingBinding where media_set in (select id from MediaSet where MediaSet.private = true) limit 1;
```