add is_default action

[YiTing623]

Add is_default field in credit_card.py, payment_method.py, paypal.py and factories.py Add set default payment method in routes.py Add test_set_payment_method_as_default and test_set_new_default_unsets_previous_default

[nedjulius]

is this ready for a review @YiTing623 ?

[YiTing623]

is this ready for a review @YiTing623 ?

Yes, please take a look when you have a moment, thanks!

[YiTing623]

few suggestions. otherwise looks good

updated. not sure if it is correct, please help review again, thanks!