Always require invoice VAT rate

Closes #1031

app/models/invoice/vat_rate_calculator.rb

@@ -0,0 +1,19 @@
+class Invoice
+  class VatRateCalculator
+    attr_reader :registry
+    attr_reader :registrar
+
+    def initialize(registry: Registry.current, registrar:)
+      @registry = registry
+      @registrar = registrar
+    end
+
+    def calculate
+      if registrar.vat_liable_locally?(registry)
+        registry.vat_rate
+      else
+        registrar.vat_rate || 0
+      end
+    end
+  end
+end