[bug 1048689] sync cta doesn't appear on whats new

Currently the Sync CTA on `firefox/31.0/whatsnew` and `firefox/31.0/firstrun` requires the UITour API to work, as it highlights the menu panel when clicked. This PR adds a fallback state that simply links the button to the `firefox/sync` page, should the UITour API not work for some reason (or JS be disabled).

This has come to light as it turns out there is a long-standing [Firefox bug](https://bugzilla.mozilla.org/show_bug.cgi?id=506446) where the preferences for things like UITour can get reset incorrectly. This bug is being escalated to be fixed now, but this should provide a fallback for the Sync CTA in the interim.

Note: you can test this works by setting `browser.uitour.enabled` to `false` in `about:config`