Pass Traffic Cop custom referrer to attribution scripts (Fixes #13593)

[alexgibson]

One-line summary

This PR fixes some existing holes in attribution data, where referrer was being lost when Traffic Cop performs a redirect.

Significant changes and points to review

I added a global Mozilla.Analytics.getReferrer() helper that other scripts can use to get referrer more reliably. It touches three places:

• Glean
• Stub Attribution
• FxA Attribution

Hopefully the tests I added can give some confidence that the value is now being passed as expected, but I also added some manual testing instructions below.

Issue / Bugzilla link

#13593

Testing

Note: test all the items below using a browser with DNT disabled.

Glean

1. Set GLEAN_LOG_PINGS=True in your .env.
2. Open http://localhost:8000/en-US/firefox/new/
3. Click on the Mozilla logo in the main navigation to go to the home page. You should be entered into a traffic cop experiment with wither ?v=1 or ?v=2 in the URL.
4. Confirm in the page ping in the web console that referrer is http://localhost:8000/en-US/firefox/new/ and not http://localhost:8000/en-US/

Stub attribution

1. Open http://localhost:8000/en-US/firefox/new/
2. Click on the Mozilla logo in the main navigation to go to the home page. You should be entered into a traffic cop experiment with wither ?v=1 or ?v=2 in the URL.
3. In the web console, run Mozilla.StubAttribution.getAttributionData().
4. Confirm that referrer is http://localhost:8000/en-US/firefox/new/ and not http://localhost:8000/en-US/.

FxA Attribution

I found this add-on useful for manual testing. With it you can override the referrer to any value you choose:

1. With a modified referrer set to https://www.google.com/, open http://localhost:8000/en-US/products/vpn/. You should be redirected into a traffic cop experiment with entrypoint_experiment=vpn-refresh-pricing in the URL
2. Scroll down to the pricing section and inspect a subscription link in dev tools.
3. Verify the link contains utm_source=google&utm_medium=organic

[craigcook]

Tested and works as expected 🪅