As part of the migration away from AngularJS, replace `$http` with
`fetch` in the OAuthClient class which handles interactions with h's
OAuth endpoints.

For testing use the fetch-mock library that is already used in h's
frontend tests. The current version (v7) has dependencies that are
written in ES6 and not transpiled, and so don't work in PhantomJS [1]. As a
workaround, use v6 of the library for the time being.

This is part of #974.

[1] We _do_ transpile our own code from ES6 -> ES5 but that transform is
    not applied to dependencies.