Allowing saving of Bancontact tokens as SEPA tokens

[wjrosa]

Fixes #3776

Changes proposed in this Pull Request:

iDEAL and Bancontact tokens are stored as SEPA tokens. Because of that, when SEPA is disabled, it is not possible to add or list saved iDEAL and Bancontact methods.

This PR adds SEPA to the list of valid payment method tokens when listing saved methods to fix the issue.

Testing instructions

• Checkout and build this branch on your test environment (add/bancontact-saved-method-as-sepa)
• Connect your Stripe account
• Set your store currency to EUR
• Enable the Bancontact payment method in settings (wp-admin/admin.php?page=wc-settings&tab=checkout&section=stripe&panel=methods). Keep SEPA disabled
• As a shopper, go to the Add Payment Method page (my-account/add-payment-method/)
• Try to add a new Bancontact method
• Confirm that the payment method is saved and is correctly listed:

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

Post merge

• Added testing instructions to the Release Testing Instructions wiki page (or does not apply)
• Added the needs docs label (or does not apply)
• Included this PR in the Release Thread scope (or does not apply)

[annemirasol]

LGTM!

I was able to save and use Bancontact tokens even when SEPA was not explicitly enabled.