[fix bug 1066865] Ignore snippets that link to about:accounts when sync is setup.

I'm not really comfortable with a half-second pause before showing snippets, especially when we're not running about:accounts snippets. Instead, how about we do what we did with the geolocation snippets, and store the value? So the first time, if there's no stored value, we just don't show the snippets, and then on subsequent times, we use the stored value and update it after the snippet has been displayed. That way, there's no delay, and only one small chance where they'd see the snippets after having signed up for sync.

Also if we do we should use `gSnippetsMap` instead of localStorage, because I think the geolocation is still using localStorage.