404 language aware

[Berdir]

This is based on #47

Todo:

• Address remaining review points from that PR
• Move code that queries the redirect_404 table to a new RedirectNotFoundStorage service (redirect.not_found_storage)
• When a redirect is created, delete matching records in that able
• More tests, e.g. for the feature above
• Clear old 404 records. Maybe a cron task with configurable time, if not requested in e.g. 4 weeks, then delete.