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.

InvenTree/order/tests.py

@@ -326,7 +326,12 @@ class OrderTest(TestCase):
                 user__id=user_id,
             )
 
-            self.assertTrue(messages.exists())
+            # User ID 3 is inactive, and thus should not receive notifications
+            if user_id == 3:
+                self.assertFalse(messages.exists())
+                continue
+            else:
+                self.assertTrue(messages.exists())
 
             msg = messages.first()