[Finished #71200960] Playing IA audio through the CDN.
[#71200960] IA url construction now in AudioFile#ia_url

AudioFileUploader#url passes responsibility down to AudioFile#ia_url
when the file is stored on InternetArchive

Spec stubs AudioFile#ia_url
[#71200960] Move the IA root url into env variable
[#71200960] Fix reference to CDN_IA_ROOT_URL