refs https://github.com/TryGhost/Team/issues/1490
With multiple newsletters, paid members can choose their newsletter preference at the time of signup via Portal. This change handles the newsletter preference via stripe checkout metadata and stores it against the member on completing signup.
refs https://github.com/TryGhost/Team/issues/1374
We cannot update payment details for members which we don't know about,
so returning and giving a successful response to Stripe is the correct
thing to do.