Since the metadata only provides a date, set the time part to 12:00:00
(UTC).
Also fix up the deprecation warning for the creation of the initial zero
last build timestamp.
This newer version is able to correctly parse the date of streamed
videos.
Also use the full `ytextract::Video` structs which should have have all
the metadata.
* 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