Build a sandbox for CRO

[hoosteeno]

Description

Implement the sandbox defined in #7505: https://docs.google.com/document/d/1RXzBVUk3LSbyBeJOesPrKcekvA5bOUBiUIiQy7frHgQ/edit?usp=sharing

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💛 Success Criteria 💛

• Sandbox is built and a) ready to port content to b) ready to send audience to

[alexgibson]

The initial pages in the sandbox will be:

• /en-US/exp/firefox/new/
• /de/exp/firefox/new/

For now we'll be hardcoding the strings for de, rather than exposing the strings through pontoon.

[alexgibson]

Depends on #7700

[alexgibson]

@hoosteeno I've implemented convert in the PR referenced above, and pushed it to demo here:

https://www-demo3.allizom.org/en-US/exp/firefox/new/

In the convert dashboard I've set up a simple A/B test (in the staging project) that changes the headline. It seems to be working ok and tracking as expected. Please take a look.

Things to note:

• I've implemented loading their script dynamically (async). This means that if the script is slow to load, or unresponsive, our visitors won't be left looking at a blank screen & loading spinner. This can mean that swapping out varation content may be visible during page load, but as long as their CDN is fast I don't think the experience is too jarring. This is a better (imho) alternative to blocking on a third-party resource.