Teach legacy download API about audio-only file formats

[rjw57]

If the legacy download API is asked for an MP3 or other audio format, it should redirect to an endpoint which is audio only. Currently the sources API takes a MIME type as a parameter. Decide if we'd like to make use of the "best source selection" logic so that we could request /api/media/{id}/source?format=audio and have it select the best audio-only format. If we do so, update the legacy download endpoint to redirect to the appropriate source endpoint if asked for an audio format.