Exclude all 404 problem reports (without referrer)

This change marks all 404 problem reports without a referrer as non-actionable,
meaning that they won't show up in FeedEx.

We're no longer capturing user feedback on the GOV.UK 404 pages (because there are better
means to capture this information, like using GA 404 page visits, or looking at CDN logs).
404 problem reports without referrer are usually most often typos by the user, as opposed to
problems with the content, and haven't proven particularly useful to content designers.