Fix email notification setting (#3832)

* Coerce setting value to a boolean

* Ignore inactive users when sending notification emails

* Only send UI notifications to active users

* Fixes for unit tests

InvenTree/order/test_sales_order.py

@@ -267,7 +267,7 @@ class SalesOrderTest(TestCase):
             category='order.overdue_sales_order',
         )
 
-        self.assertEqual(len(messages), 2)
+        self.assertEqual(len(messages), 1)
 
     def test_new_so_notification(self):
         """Test that a notification is sent when a new SalesOrder is created.