Select the well-supported, almost always available MP4 container format
that is directly available (so no HLS or DASH). This unfortunately
does reduce the bitrate to 64 kbps.
* Add a `backend` module `Backend` trait and necessary abstract types
* Refactor handlers to use the back-end abstraction
* Directly serialize to URLs where necessary in Mixcloud back-end
* Require `serde` feature for the url crate
Paul van Tilburg