Track funnel events with Segment.io

* Base approach on http://robots.thoughtbot.com/segment-io-and-ruby.
* Add `utm_` campaigns as properties on user creation so cohort analysis can be
  done on a per-campaign basis later on.
* Use special Segment.io `revenue` property and special `campaign` context.
  https://segment.io/docs/tracking-api/track/

https://trello.com/c/gwFd8KCN
Interesting. I'd missed the subtle dependencies in my first review, so overall I think the new approach is much clearer.