Make PoP start and end dates inclusive.

Also removes the clock class.

Makes PoP date ranges inclusive such that a task order with:
-  a start date on or after the current date
and
- an end date on or before the current date
should be considered valid.

This commit also removes the Clock class. This class had two methods as
shortcuts for common uses of pendlum functions. But it wasn't being used
in very many places, and it took up about the same space as

    from pendulum import today()
    ...
    today(tz="UTC").date()

If we want to add this back in, it might be a good idea to extend it for
other time functions we have sprinkled around, like the random date
functions in our tests

tests/models/test_task_order.py

@@ -87,17 +87,28 @@ class TestTaskOrderStatus:
     @patch("atst.models.TaskOrder.is_completed", new_callable=PropertyMock)
     @patch("atst.models.TaskOrder.is_signed", new_callable=PropertyMock)
     def test_active_status(self, is_signed, is_completed, start_date, end_date):
-        # Given that I have a signed TO and today is within its start_date and end_date
-        today = pendulum.today().date()
-        to = TaskOrderFactory.create()
+        today = pendulum.today(tz="UTC").date()
 
-        start_date.return_value = today.subtract(days=1)
-        end_date.return_value = today.add(days=1)
         is_signed.return_value = True
         is_completed.return_value = True
 
+        # Given that I have a signed TO and today is within its start_date and end_date
+        to_1 = TaskOrderFactory.create()
+        start_date.return_value = today.subtract(days=1)
+        end_date.return_value = today.add(days=1)
+
         # Its status should be active
-        assert to.status == Status.ACTIVE
+        assert to_1.status == Status.ACTIVE
+
+        # A period of performance's start and end dates are inclusive, so a TO
+        # should be active on its start and end dates
+        to_2 = TaskOrderFactory.create()
+        start_date.return_value = today
+        end_date.return_value = today
+        is_signed.return_value = True
+        is_completed.return_value = True
+
+        assert to_2.status == Status.ACTIVE
 
     @patch("atst.models.TaskOrder.end_date", new_callable=PropertyMock)
     @patch("atst.models.TaskOrder.start_date", new_callable=PropertyMock)