Enable the use of saved SEPA payments

[superdav42]

When working on #1452 I found that checking the " Save payment information to my account for future purchases." did not work for SEPA payments. I have not investigated what the issue is.
We need to allow customer to save their IBAN so they can more easily checkout in future orders.

Should probably be done after #1468

[c-shultz]

It looks like this could be a good one to save for sprint 2. SEPA will work without this, right @superdav42 ?

[superdav42]

Yes, it will work but we need to either disable the option or fix it before we try to launch SEPA. I'm guessing it won't take much to fix it but I haven't tried yet.

[harriswong]

Notes:

• In order for the "Save payment information..." checkbox to show, make sure you are logged in as a customer (not inside wp-admin).
  
• If we have both "Credit Card" and "SEPA" payment methods enabled while store's currency is USD, it will fail with this error:
  
  Change currency to EUR and it will be okay. Ideally, if the store is set to USD, "SEPA" should be hidden but I think this is something to be handled in Settings. We should test both USD/EUR when we roll this out.