Determine whether user is already signed up for the given newsletter

If a user has already signed up for an email newsletter, we don't want to ask them to sign up again.

We should expose a method on Bridget that allows us to determine whether a signed-in user is signed-up for a newsletter. If not, we can show them the newsletter sign-up form

Start at the Bridget interface and the iOS and Android native bridge entry points; trace how a signed-in user’s newsletter state could be queried. Done means Bridget exposes a way to determine whether the user is already signed up, so the caller can decide whether to show the newsletter sign-up form.

Written by the indexing model from the issue text.