FxA web flow (bug 1033694)

this if statement feels wrong because it relies on *implied* FxA activation logic. I suggest adding a data attribute in Webpay [here](https://github.com/mozilla/webpay/blob/master/webpay/spa/templates/spa/index.html#L12) like ``data-fxpay="true"`` so that you can rely on *explicit* FxA activation logic. You can require ``lib/utils`` and get it from ``utils.bodyData.fxpay``.