Original referrer is lost when traffic cop redirect occurs

[alexgibson]

Description

When a traffic cop redirect occurs, the original page referrer is lost, and replaced with the canonical URL where the redirect initiates. We account for this in GA using some custom code here: https://github.com/mozilla/bedrock/blob/main/media/js/base/core-datalayer-page-id.js#L33

It would be useful to preserve this original referrer so that other scripts can reference it, to make sure that the original referrer is preserved more accurately in other places that might use it (Stub attribution, FxA attribution, Glean etc).