Use the checkURL func and remove magic

Use the checkURL func and amend settings for tests + add a flow test.
I think you need to white list sites, not hosts. E.G. ``validSites: ['http://mozilla.bango.net', 'https://mozilla.bango.net', 'https://marketplace.mozilla.org']``. The reason is because we will always need to allow the ``http`` bango redirect in production but we do *not* want to allow an ``http`` redirect to mozilla domains in prod.