Aff Pro Migration enchancements + more

Made a few enhancements to aff pro migrations : only migrate affiliates that had existing referrals and hits (otherwise the whole user database was migrated)

Only show the Aff Pro migration button if the plugin is activated

Fixed issues with default values for inserting new affiliates

Added a ignore zero referrals setting under misc tab. If a payment went through with zero total (perhaps from a discount code) then the referral is not logged (as the referral fee would be zero)