Currency and PaymentMethod disabling should have an effect

• PaymentMethods' enabled status is an AND combination of their own enabled property and the enabled property of its Currency

When a PaymentMethod is disabled (it itself is disabled or its currency is):

• Existing Tills should not be allowed to accept payments via this Payment method
• New Tills, if created with an existing TillPaymentOptions, should still be created with a TillMoneyCount of this PaymentMethod, and they should respect the previous item