Direct download support

[tsani]

We currently only support youtube-dl downloading, but if a user has a link directly to an audio file, we should be able to download that. Also, links to zip files or other archives?

[tsani]

This should mean refactoring the /tracks endpoint. I think that this endpoint itself should support POST requests to create new tracks, with the request body specifying the specific strategy to use for acquiring the track (be it direct download from a URL, youtube-dl invocation, track supplied as base64, whatever).