The `pubDate` field of an item in the feed is meant to be time the item
was published. It should not be bumped if the item is updated in the
backend.
* Introduce a new `published_at` field on `Item`
* Update the Mixcloud and YouTube backends to fill this field
* Use the `published_at` field on `Item` for the `<pubData>` item
subelement
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
