Add in link tracking for 'Change Answer'

- This pull request is the result of a conversation with @antimega about what's useful to track when a user passes through a smart-answer flow.
- We will now track whether a user clicks on *'change answer'* or on *'start again'* links within the flow.
- When these links are clicked we'll capture the majority of the current url, or the entirety of the 'change answer' href, this way we should be able to figure out:
  - type of item clicked (start again or change answer)
  - name of the smart answer flow, e.g. 'overseas-passports'
  - location within the flow of this click, e.g. 'overseas-passports/y/sweden/renewing_new'
- We also remove all references to `'.undo'`, which was previously used to denote a 'change answer' link