If the 'url' value is not provided, grab it from the request referer

If a POST occurs to /feedback that doesn't explicitly contain a
parameter for 'url', we should fill this from the request referer
where possible.
Add a comment explaining the HTTP referer edge case
Merge pull request #36 from alphagov/accept-feedback-without-url-value-set

If the 'url' value is not provided, grab it from the request referer