Newsletter recovery page should post directly to basket (Fixes #11963)

[alexgibson]

One-line summary

Migrates the newsletter recovery form from Python to JS, which now posts directly to Basket.

Significant changes and points to review

Did I miss any edge cases that were in the back-end code?

Issue / Bugzilla link

#11963

Testing

• http://localhost:8000/en-US/newsletter/recovery/
• http://localhost:8000/en-US/newsletter/opt-out-confirmation/