Auto-enroll new US newsletter subscribers to First Thing / Sat Edition

**Overview**

Following this ticket: https://github.com/guardian/dotcom-rendering/issues/15877 we need to auto-enroll new US newsletter subscribers into First Thing and Saturday Edition, as well as to an ad-hoc list will be used to send out a few (tbc) non-newsletter editorial emails a month to.

**Scenarios**

Brand new subscriber:
Auto-enroll to both First Thing, Saturday Edition and the ad hoc list

Existing subscriber signing up to new newsletter:
Auto-enroll them into First Thing (if not already enrolled), Saturday Edition (if not already enrolled) and the ad hoc list

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Ad hoc list will get editorial emails sent 2-3 times/month. Not full newsletters.