Instead of hardcoding the url sent to enketo (which created all kinds of headaches in spinning off
formhub instances for mvp + ossap), made it configurable in settings, which addresses the issue
@MartijnR raised: https://github.com/SEL-Columbia/formhub/issues/1331
Also restored alternative url under TESTING_MODE, since, according to @ribalba it is necessary
(though changed the test url in question to something we know will be available going forward,
not some random site which is now a spam blog, and may not be up in the future).