Fix issue with filename history

https://trello.com/c/iSqthcCg/71-bug-404-error-when-opening-attachment

As we were testing something with SpecialistPublisher, we saw that files with spaces in their names were always 404ing. After some investigation by myself and @evilstreak, we saw that the code merged in https://github.com/alphagov/asset-manager/pull/31 was generating a filename_history before Carrierwave had a chance to convert spaces to underscores, so the URL would always look for the_document.pdf and the filename history had 'the document.pdf'.

As we were looking into this, we also noticed that any file uploaded before the merge would have a blank filename history which would cause them to 404. 

The commits in this PR only update filename history when the file is replaced and the `valid_filenames` method looks in both the history and the current filename.