Client handles redirect + auth

[lovelydinosaur]

Builds on #544

Drops redirect and auth from being middleware, and just handles them on the Client instance.

[lovelydinosaur]

[florimondmanca]

@tomchristie The only thing itching me on this is that we’re back with having lots of redirect utility helper methods on the Client.

They used to be neatly encapsulated in the middleware class.

Any chance we can keep redirect-as-middleware and the benefits from the refactoring here?

[lovelydinosaur]

I'm not really fussed about it - they're neatly demarked. (In the same way as the merge_<...> methods.)

I'd far rather we have a mostly linear chain of control to follow through, like this.