Add canBeRefunded to webshop GraphQL schema

This uses property ParkingPermit.can_be_refunded, so we can check logic
in frontend if a refund is needed when ending or editing a permit.

Refs: PV-734

parking_permits/schema/parking_permit.graphql

@@ -91,6 +91,7 @@ type PermitNode {
   primaryVehicle: Boolean
   monthsLeft: Int
   currentPeriodEndTime: String
+  canBeRefunded: Boolean
   canEndImmediately: Boolean
   canEndAfterCurrentPeriod: Boolean
   hasRefund: Boolean