Implement A/B testing for footer newsletter sign up

[data-sync-user]

Note: First confirm if DET team would like to A/B test the Newsletter Sign Up form in the footer, but my guess would be yes.

If so, we can do so by copying the “Donate Banner A/B Testing” functionality from the legacy site.

This would look like:

• Create a FooterNewsletterSignup page, similar to the DonateBannerPage from the legacy site. This page has only one field and its to select a newsletter sign up snippet.
• Update our BasePage model with a way to add the NewsletterSignup that was selected’s info to our template context for rendering.
• Copy over any other functionality needed.

Here is the Sitewide Donate Banner A/B testing PR for reference if needed: https://github.com/MozillaFoundation/foundation.mozilla.org/pull/12927

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