Upgrade to elm-http-builder supporting elm/http 2.0

[Natim]

It is far from working but I will leave it there for now, if someone wants to tackle a next step.

Thing I discovered:

• [x] We need our own expectJson layer to handle kinto errors.
• [x] We need to change the API to pass a message all along.
• [x] withCredentials handling might be removed I guess.

[Natim]

I'm going to close this and start again with the new elm-http-builder release ported to elm-http/2.0