Add global option to bypass a credit card requirement

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We often pre-register users, so sometimes a user may have a pin and a paddle_number but not a credit card on file. For some auctions where the guests are vetted that's OK. Add a global option to allow bypassing the cc prompt when a user doesn't have a cc on file.

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Another possible work-item is that the auction dictates whether a cc is required, https://trello.com/c/GaTuNQDv/877-auctions-should-dictate-whether-cc-is-required-or-not-for-registration, but I can already see people wanting this "only for people admitted to the room".