[fix bug 1003048] Tour pages should try to detect if Mozilla.UITour APIs are working

This PR adds a safety measure that ensures the tour should only start if the super-power API is working. If not, the user should just get the regular web-page.

This moves the code that starts the tour inside the API callback to `getConfiguration`, so it only gets called when the callback is returned.

Also includes a minor alteration when calling `onCloseTour` on the last step in `browser-tour.js`