Purchases: Make credit card page more flexible

[gziolo]

This is a follow up for #7484.

In this PR I added another couple of refactorings:

• we have now 2 entry React components to distinguish Add Card Details (<AddCardDetails />) and Edit Card Details (<EditCardDetails />) pages
• <PurchaseCardDetails /> was extracted and contains shared logic for Card components that operates on purchases
• .../payment/edit url was updated to better reflect functionality of the page - .../payment/add

With that in place we should be ready to work on new flow for Connect for WooCommerce.

Testing.

1. Go to Manage Purchases page (http://calypso.localhost:3000/purchases).
2. Select a purchase paid with credit card.
3. Click on Edit Payment Method link.
4. You should see loading placeholder when page is loading (you can refresh a page if you missed loading step).
5. When page is loaded you should see a form with a cardholder name provided.
6. Play with the form to make sure it works as before.
7. You can also try to click back button while validation is pending to make sure isMounted change is working properly. There should be no warnings on JS console complaining about incorrectsetState usage.
8. Go back to Manage Purchases page.
9. Pick a purchase paid with credits.
10. Click on Add Payment Method link.
11. You should see loading placeholder when page is loading (you can refresh a page if you missed loading step).
12. When page is loaded you should see a form with no fields filled in.
13. Play with the form to make sure it works as before.

Test live: https://calypso.live/?branch=update/improve-credit-card-page

[gziolo]

I'm receiving the following error when submitting a AUTH_DECLINE card:
Warning: setState(...): Can only update a mounted or mounting component. This usually means you called setState() on an unmounted component. This is a no-op. Please check the code for the CreditCardPage component.

@prettyboymp It's fixed now with 3105c04. We should do much better job when handling async request in Calypso. I guess it's wider issue.

[janm6k]

Looks good 👍