feat(oauth): Support for URN redirects


Fixes:  https://github.com/mozilla/fxa-content-server/issues/1585

@ckarlof @zaach r?
Or, you could use the `url` module and do something like `url.parse(uri).protocol`, but that gets ugly.
```js
["http:", "https:"].indexOf(url.parse("http://foo.bar").protocol) !== -1;
```