Check permission to play audio before adding to playlist

Issue Summary

Currently, audio_feeds checks the permission to download before allowing an audio node into its playlist. This seems wrong. Instead, it should probably check the permission to play the audio, as playlists are meant for playing rather than downloading. Site owners will often want to hide the download links by disabling the "Allow file downloads" option on audio nodes, but will still want those nodes to be listed in playlists.