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
Paul van Tilburg