Track when a user wants to change a previous answer

It's possible, but very difficult, to infer this from GA flows. Explicitly track it as a custom event so we can make design decisions based on data.

* Match the existing (but broken) GOVUK.Analytics.Trackers event convention
* Using slug as action will allow looking at smart-answers by change rate
* Slug extraction is a little brittle, but:
** This is a short lived change to get some data
** It's based on existing logic that is working correctly.
Merge pull request #847 from alphagov/change-answer-analytics

Track when a user wants to change a previous answer