Create Stripe customer on-the-fly in createPortalLink to fix #479

[christiangenco]

Right now Stripe Customers are created on-the-fly with the createCheckoutSession callable function but not with the createPortalLink callable function.

This PR copies the Stripe Customer on-the-fly creation in the createCheckoutSession callable function to the createPortalLink callable function.

More details are in my issue #479.

fixes: #479

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1 out of 3 committers have signed the CLA.

✅ dackers86
❌ jsteele-stripe
❌ christiangenco
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