Allow free allowance to be set to 0

We want to be able to set the free allowance for a service to 0, but the
form was not allowing this - it gave an error message of `Cannot be
empty`. This can be fixed by changing the WTForms validator from
`DataRequired` (which coerces 0 to falsey) to the `InputRequired`
validator.

tests/app/main/views/test_service_settings.py

@@ -3800,6 +3800,7 @@ def test_should_show_page_to_set_sms_allowance(
 
 @freeze_time("2017-04-01 11:09:00.061258")
 @pytest.mark.parametrize('given_allowance, expected_api_argument', [
+    ('0', 0),
     ('1', 1),
     ('250000', 250000),
     pytest.param('foo', 'foo', marks=pytest.mark.xfail),