Add universal analytics snippet as test
Only use cross domain analytics on IER transactions

This is intentionally scoped to IER in a simple fashion as we decided not to build
a generalised solution until we have more than 1 example of user needs for this
kind of tracing, eg, do we have to support non-GA solutions, non-universal analytics?

Keep it simple for now.

https://www.pivotaltracker.com/n/projects/1008986/stories/74213668
Tweaked GA linking params, as per the guide from Pete on IER
Test that we only include analytics JS on regsiter to vote
Merge pull request #643 from alphagov/register-to-vote-ga-test

Include IER analytics JS on register-to-vote start page