feature(snippet): add survey snippet for some en users

<del>Don't merge until we get approval on the strings.</del> Ready to go, pending review of latest changes by @pdehaan or equivalent LV 99 QA mage. Fixes #1206.
Cool, that's what I was wondering [here](https://github.com/mozilla/fxa-content-server/pull/1225#discussion_r13746382). I'll update it.