Set up default currency accounts automatically (#72)

backend/product/payments/model.go

@@ -53,6 +53,7 @@ func (*Record) Collection() string { return "payment_quotes" }
 
 type Provider interface {
 	Namespace() string
+	EnsureAccounts(context.Context, string) error
 	Accounts(context.Context, string) ([]Account, error)
 	Quote(context.Context, string, Request, []Account) (Quote, error)
 	Execute(context.Context, string, string) (RemoteOperation, error)