Payment step cleanup

[elia]

Summary

Removes the two tier radio buttons to select between wallet payment sources and filling in from scratch.

It's predicated under the following assumptions:

• The previous system was confusing, with its interacting radio buttons which were presented on both sides without much consistency.
• The list gives precedence to wallet payments as they require less clicks and were already the default choice if present.
• If no wallet payments are available the first payment method is selected, according to the order set in admin.
• Since to the customer they're all payment methods they're treated as a single radio list.
• Both types of payments are now visually consistent, better communicating all the options to the customer.
• The payment details are wrapped in a fieldset that gets disabled whenever a payment method is unselected, this also works as a hook for the payment provider specific implementation (e.g. solidus_stripe) to react properly when selected/unselected.

This layout and decisions are also informed by multiple real world stores that got their payment step redesigned.

• depends on #301

Before

Show original behavior

https://user-images.githubusercontent.com/1051/219420532-c91ffe24-ea19-488b-84f4-8ba45145195e.mov

After

https://user-images.githubusercontent.com/1051/219440642-112350bc-1893-459c-b4fd-c8f24aba4e44.mov

Checklist

Check out our PR guidelines for more details.

The following are mandatory for all PRs:

• [x] I have written a thorough PR description.
• [x] I have kept my commits small and atomic.
• [x] I have used clear, explanatory commit messages.

The following are not always needed:

• 📖 I have updated the README to account for my changes.
• 📑 I have documented new code with YARD.
• 🛣️ I have opened a PR to update the guides.
• ✅ I have added automated tests to cover my changes.
• 📸 I have attached screenshots to demo visual changes.

[kennyadsl]

@elia still not reviewing but... a "before" screenshot would be helpful to quickly navigate what's changing, especially when the description of the PR is empty. 😬

[elia]

@kennyadsl yes 💯 agree, will probably do a short video demonstrating how broken it is