Select shipping address as returning customer: consistent button names

[kqmpetenz]

Reproduce:

1. login with account
2. make sure the account already stored a shipping address
3. put a random product in cart and open cart
4. click "Proceed to checkout"

Issue: Button name is inconsistent

• button name on checkout pages for guest order: "Continue"

• button name on shipping address page for returning customer: "Continue"

• button name on billing address page for returning customer: "Use this billing address"

• button name on payment page for returning customer: "Use this credit card"

[freddyka]

Issue not fixed for poster v2.1.