Checkout: Rearrange Steps

[CrashKid]

Since getSupportedCheckoutMethods() often has a dependency on a valid billingAddress and/or shippingAddress, we want to change the order of the checkout process steps (back) to the common way.

While the current flow looks like this:

1. Select Payment Method
2. Select Shipping Method
3. Add Billing Address
4. Optionally add Shipping Address

We want it to become this:

1. Add Shipping Address (yes, better ask for the Shipping Address first)
2. Optionally add Billing Address
3. Select Payment Method
4. Select Shipping Method

Payment and shipping methods will stay disabled, until the required address fields are filled in and valid.

What about PayPal Express Payment?

Paypal Express takes the address data which is stored in the user's PayPal account, so there's no need to ask the user for any address information if she plans to checkout via PayPal Express.
For this reason we want to exclude the PayPal Express payment method from the payment methods select box (which in the new design comes after the address fields) and place it on top of the checkout page (as in the cart drawer).

What about customer score checks?

Any customer score check will only run when the user has selected a payment method that requires checking.