Can only update a competition's currency code if there are no unrefunded payments

[dunkOnIT]

Addresses #1356 - in draft because I've asked WCAT if this is something that's still needed.

If this is needed, then I think this implementation may be a bit thin. If we get the mega-edge-case of a user having been over-refunded, and another user still having a not-yet-refunded payment, the total amount paid for a competition will be $0 even though there are still two unresolved payments.

As such, it might be better to find a registration with a non-zero sum of its registration_payments

Also - it seems like we don't track the overall status of a payment (eg, whether it has been refunded) - we only infer this by summing registration_payments related to a given registration.

[gregorbg]

Hm, the original issue also mentions another interesting edge case: Simple questions like "How much income does this competition have?" also rely on a (competition-)global currency. So even if there are no unrefunded payments, you would still sum up existing payments and then treat them as the new currency.
TL;DR I think we should just straight out forbid changing the currency as long as payments exist.

[gregorbg]

Another alternative would be to only allow updating the currency if all payments are fully refunded. If I have ten payments worth 1500 Something-Dollars each, and I refund every last one, then the sum evaluates to zero, which pretty much has the same meaning in any currency.

[dunkOnIT]

Another alternative would be to only allow updating the currency if all payments are fully refunded. If I have ten payments worth 1500 Something-Dollars each, and I refund every last one, then the sum evaluates to zero, which pretty much has the same meaning in any currency.

I think this is what I'm already doing? If you check the diff this is my logic to check for "unrefunded" payments:

registrations.joins(:registration_payments).sum('registration_payments.amount_lowest_denomination')