Complete the magic of the ajax absolute URL

The theory for doing this is:
* In development we do not use the `ajaxUrl` property so all URLs aren't `crossOrigin`.
* When releasing to `code` / `prod`, we are now magically put into a `crossOrigin` situation.

This way when we force you into that situation, we make sure we make the necessary accommodations.
We could do a check on `if (location.host !== url.host)` but seems overkill and it works without it.

Now we don't have to have something working locally, that breaks when we release it.

Fixes #3534 due to [this](https://github.com/guardian/frontend/blob/master/common/app/assets/javascripts/bootstraps/football.js#L247).

![Magic hands!](http://cdn.theatlantic.com/static/easel/images/galleries/120051_romney_sparkles_jazz_hands.gif)