Add standardized GA events to WNP templates

[alexgibson]

Description

There's an ask that we always include the following GA events in every WNP:

• Firefox default / not default
• Fxa Signed in / not signed in

Rather than having to add this manually to every WNP, it would be much easier if we standardized them in one script that can be included in every WNP without having to repeat the same thing over and over again every release.

---

Success Criteria

• Script added to include these events by default for each WNP.