[IMP] payment_stripe: flow changes

The previous commit introduced some changes for SCA/PSD2 but some things
were missing; most notably, registering payment cards in a s2s flow used
the old Charge API that is not SCA-ready.

This commit uses the SetupIntent API instead, allowing to register the
card prior to payment. The global flow is described in the Stripe
documentation in more details: https://stripe.com/docs/payments/cards/saving-cards

Unfortunately, given the complete flow of payments, it is not possible
to use the PaymentIntents API since this would require having the
reference of the transaction before it gets created (the TX is created
after the form submission, but the token must already exist at that
point). Since this is meant to be backported, this changes are kept
minimal in this feature branch. The PaymentIntents API would have been
better since registering a card during a payment makes it more likely
that future off-session transactions will not require an authentication
step. We'll have to live with that for now; it's difficult to know
exactly how this will affect customers before PSD2 goes into effect on
september 14.

Another branch might completely change the flow soon-ish to a more
asynchroneous model, although it may miss the v13 freeze date -_-

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