Refactor core.GetModeratorFee to use repo.Profile

[placer14]

core.GetModeratorFee accesses internal profile state which is already nicely exposed via repo.Profile.GetFixedFee.

• It would be good to refactor this to use existing repo.Profile.GetFixedFee
• Bonus: Expose a new function which provides (repo.Profile).GetModeratorFee(orderTotal *CurrencyValue, cc *CurrencyConverter) (feeTotal *CurrencyValue, error) which takes in the orderTotal (in the currency of the order) and converts the local moderator state into a feeTotal converted to match the same currency/divisibility of the orderTotal.

Note: This depends on *CurrencyConverter being delivered as part of #1949