Exit tracking uses slug in data attribute if available.

This change makes exit tracking use the slug in the
data-transaction-slug attribute on the link if it's available.  If not
it will fall back to parsing the URL.

This will fix a number of issues we're seeing with exit tracking where
it's not able to get the correct slug from the URL (e.g. mixed case
slugs, pages from the google cache etc...)
Merge pull request #310 from alphagov/fix_exit_tracking

Exit tracking uses slug in data attribute if available.