Mark redirect URLs as 'seen'; add to TTLHashSet

Mark URLs returning a HTTP 3XX redirect status as 'seen' in the
Redis-backed TTLHashSet so that we don't try crawling them again until
the TTLHashSet expiry time (currently 48 hours) has passed.

Also, change the if block to use `switch` for readability.
Change INFO log message to ERROR
Merge pull request #59 from alphagov/76585878-mark-redirect-urls-as-seen

[#76585878] Mark redirect urls as seen