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
This was suffering from Connascence of Position as it needed to be called before `analytics` because `analytics` was relying on `current_user`, which is drawn from `session[:remember_token]`.

In the use case of signing in or signing up, it doesn't make sense to rely on the `analytics` helper. We can instead construct `Analytics` instances by hand here.

So yes, good idea. Done in ac60c3c.