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.
@johngruen - I reversed the logic here and changed `MARKETING_PERCENTAGE` to `SURVEY_PERCENTAGE` Should 10 or 90% of people see the survey?