Adjust slug name sent with service feedback

Before this change, service feedback submissions report the slug
to be '/done/<transaction-slug>'. For the purposes of collating
and reporting on the performance of transactions, it's more useful
if the slug is sent as '<transaction-slug>'.
Send originating URL along with service feedback

For the purposes of exploring the service feedback, it's useful
to have the originating URL as the anchor. This change passes this
information to the 'feedback' app.
Merge pull request #514 from alphagov/service-feedback-improvements

Service feedback improvements