Enable the use of saved SEPA payments

[harriswong]

Fixes #1471

Changes proposed in this Pull Request

• Introduced a SEPA token class to "save SEPA payment details". Details are stored in
• Introduced a Payment_Method class for card, sepa_debit. Planning on adding sofort and giropay later.
• Customers can now save SEPA payment information by clicking this checkbox:
  
• Customers can now use saved SEPA payments to checkout:
  

Concerns

• SEPA token class name overlaps existing plugin's.

Testing instructions

Similar to the setup in #1537

1. Set store currency to euro
2. Enable feature flag wp option, update _wcpay_feature_sepa to 1
3. Enable SEPA in checkbox at bottom of /wp-admin/admin.php?page=wc-settings&tab=checkout&section=woocommerce_payments
4. Add item to cart and go to checkout
5. You should see this screen, type in SEPA debit card number, click "Save payment...". Checkout
   
6. Checkout should be successful. Purchase another item. Now select the SEPA payment we just saved,
   
7. The checkout should proceed successfully with the saved SEPA card.

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• Added changelog entry (or does not apply)
• Covered with tests (or have a good reason not to test in description ☝️)
• Tested on mobile (or does not apply)

[vbelolapotkov]

Hey @harriswong ! Have you requested a review for this within your team? There is a recent change in approach discussed in slack (p1618348326004200-slack-C01JC3PBZL6). So @Automattic/gamma will be happy to review it at the final stage 🙂

cc @c-shultz

[harriswong]

Hi @vbelolapotkov ! Nope, not yet. I will wait for our team to review first, then I will add gamma back to the final stage. Thanks!

[brezocordero]

I have tested different scenarios and all worked:

• Save SEPA for existing customer
• Save SEPA for guest customer creating account on checkout
• Payment using the saved method
• Verify CC was still working

The code is also good, the structure is similar to what Stripe plugin is doing, which makes sense.

A couple of points:

• Compatibility with Stripe plugin. Thanks for pointing it out. We can create a separate issue but I think we need to address it.
• On My Account: Add SEPA Payment method, delete payment method and set as default will be handled on Fix Add payment method on My Account for SEPA #1538

[kalessil]

Thank you for enabling the saved SEPA payment @harriswong! Worked like a charm 🎉 !

Architecture-wise looks also good. I left a few comments, which should make the changes consistent with the existing codebase, but overall great job here!

[kalessil]

Doc-block alignment.

[kalessil]

I meant to add one more space in $item (before 'Individual'), so the descriptions get aligned.

[harriswong]

Woops. Fixed. Added spaces so variable and description are aligned vertically.

[kalessil]

On it 👀

[kalessil]

@harriswong: LGTM,

PS: See also my responses on doc-blocks.

[harriswong]

@kalessil Thanks for taking another look. I added some spaces between the variable name and description. Now it should be vertically aligned.

[kalessil]

@harriswong, thanks! Let's merge it then 🎉