Display a Name on card input on checkout payment step

This is my proposal to fix #2486

I replaced the first_name / last_name hidden inputs for a ``name_on_card`` text input. I find it odd that the ``Spree::CreditCard`` has first and last name when I believe only ``credit_card#name`` is actually used. I think those two columns could be removed in favor of just one ``name`` column. So far I didn't touch migrations. I'd like to get an approval of the idea before moving any further with this. cc @GeekOnCoffee @radar