This way handlers don't need to do case matching on backend ID strings
anymore.
* Rename `backend` to `backend_id` where we have a backend ID
* Add `get` function and `Backends` enum to the `backend` module
* Add a depend on the `enum_dispatch` crate
* 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
* Enable the `async` feature for the `cached` crate
* Make the types that we cache implement `Clone`
* Rename the argument of mixcloud::redirect_url` because of this issue:
https://github.com/jaemk/cached/issues/114
* 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
