Bug 899837 - Don't always say thanks on newsletter/updated

Only say thanks on newsletter/updated page if the page isn't
already saying something more specific, like "We're sorry
to see you go" or "Thanks for telling us why you're leaving".
Merge pull request #1138 from dpoirier/bug-899837-tweak-newsletter-updated-thanks

Bug 899837 - Don't always say thanks on newsletter/updated