Convert radios on SMS prefix page

Includes changing form.enabled to use
OnOffField, for consistency with other on/off
fields.

OnOffField's data is a boolean, not a string, so
some of the logic using it needed to be changed.

tests/app/main/views/test_service_settings.py

@@ -4450,22 +4450,21 @@ def test_show_sms_prefixing_setting_page(
     )
     radios = page.select('input[type=radio]')
     assert len(radios) == 2
-    assert radios[0]['value'] == 'on'
+    assert radios[0]['value'] == 'True'
     assert radios[0]['checked'] == ''
-    assert radios[1]['value'] == 'off'
+    assert radios[1]['value'] == 'False'
     with pytest.raises(KeyError):
         assert radios[1]['checked']
 
 
-@pytest.mark.parametrize('post_value, expected_api_argument', [
-    ('on', True),
-    ('off', False),
+@pytest.mark.parametrize('post_value', [
+    True,
+    False,
 ])
 def test_updates_sms_prefixing(
     client_request,
     mock_update_service,
     post_value,
-    expected_api_argument,
 ):
     client_request.post(
         'main.service_set_sms_prefix', service_id=SERVICE_ONE_ID,
@@ -4477,7 +4476,7 @@ def test_updates_sms_prefixing(
     )
     mock_update_service.assert_called_once_with(
         SERVICE_ONE_ID,
-        prefix_sms=expected_api_argument,
+        prefix_sms=post_value,
     )