* 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
* Derserialize the paging information
* Parse each next URL; handle URL parse errors
* Use a default page size of 50; pass offset 0 to count by item index
* Use the `thiserror` crate to make our own error type
* Implement Rocket's `Responder` type for the error type
* Adjust all methods to use the error type
* Small documentation tweaks