Remove all references to 'undo' links

The original commit that added tracking to these links appears to have gone in in April as a temporary measure to support some design reckons. This commit proposes to take out the link tracking and css references to 'undo', as we no longer seem to have the concept of an 'undo' link in the markup.

Original ref from `git log -p -S 'This is short lived tracking'`:

```git
commit cfefadf12a3844bb03838b9b2d9cd98fe2239bb6
Author: David Singleton <david.singleton@digital.cabinet-office.gov.uk>
Date:   Mon Apr 28 13:38:39 2014 +0000
```
Change getCurrentSlug to pass back more information

- Previously if we had a URL that was e.g. `http://smartanswers.dev.gov.uk/overseas-passports/y/afghanistan/applying` `getCurrentSlug()` would return `"overseas-passports"`
- Now from that URL we will return `"overseas-passports/y/afghanistan/applying"`
- This is based on a conversation with @antimega about the actual usefulness of what we're capturing. With this level of fidelity we capture not only that someone clicked on a link inside the `overseas-passports` flow, but the point in the flow that they decided to change their answer.
Indentation cleanup

- Not sure how this ended up over here, it shouldn't be
Use `getCurrentPosition` for start again, pass href for 'change answer'

- Adds click tracking back in for changing of answers.
- Tracks the href of the change answer link in its entirety, e.g. `"/overseas-passports/y/?previous_response=afghanistan"`
Merge pull request #1375 from alphagov/feature/change-link-tracking

Add in link tracking for 'Change Answer'