Introduce currency setting

[gdlcf88]

• A new setting named "CurrencyCode" in the Orders module.
• It specifies a valid currency code for stores, tenants, and the host.
• It will finally fall back to "USD" if the value doesn't set.
• Check whether the currency code of OrderLines is equal to the value from the setting.
• The store-side setting value implementation depends on #174.