Organizer Payment Info: Add invoice currency to user account

[mariobehling]

In https://eventyay.com/account/billing/payment-info we need an option for invoice currency. The currency chosen should be the currency that the system uses to calculate the monthly invoice.

Please offer the following currencies as an option: EUR, SGD, USD

[iamareebjamal]

I think this should be de-escalated from urgent. As we don't have a single invoice for multiple events, we don't need to convert the amount into a single currency and it would require integrating a currency conversion tool and more work. Secondly, as we will not be using this currency currently, this will only confuse the users when they see invoice is not in the same currency they chose in the billing form

[mariobehling]

Sorry, clicked Urgent by mistake.