fixes https://github.com/TryGhost/Ghost/issues/16057
Briefly, Ghost created two Customer objects via the Stripe API when an
existing subscriber would upgrade to a paid subscription, one in an API
call to create the Customer and then a second as a side effect of an API
call to create a Checkout session for the user. The fix is passing the
reference to the Customer object to the API call to create the Checkout
session; Stripe will no longer redundantly create a Customer object in
this case.
This largely impacts the owner's experience of the Stripe Dashboard; it
will correct their new Customer count (going forward) and make searches
for users by name or email address return one responsive object which
has the actual subscription in it versus returning two and forcing them
to look in each to e.g. refund a transaction or similar.
559ca9d866