Create Payment Methods for Stripe

[nerrad]

Using the api introduced in #1349, this pull will create the ExpressPaymentMethod and PaymentMethod integrations for Stripe. Some notes:

• should use as much of the existing stripe extension logic as possible (so will need to detect if the extension is installed).
• should be created with a goal of moving this logic to the actual payment method extensions themselves (possibly before release). We may need to talk more about this.
• Will need to expose icons (on a yet to be built api) for the cart payment icons. (see Cart: Create api for payment methods to expose card icons. #1472)

Here's an updated mockup provided by @LevinMedia that shows a few changes:

• label for stripe tab is Credit/Debit Card
• Credit card icons get moved to below save payment information checkbox.
• Saved payment methods are moved outside of the PaymentMethod component and are a part of the payment step (saved payment methods are provided by checkout).
• Payment Method component only gets displayed if there are no saved payment methods or "Use a new payment method" is selected.

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