Fix attachment filename and title scrubbing

Fixes https://www.pivotaltracker.com/story/show/59031842

**For discussion**: We have a [simple test](https://github.com/alphagov/whitehall/blob/fix-attachment-scrubbing/test/integration/sanitise_db_test.rb) for this script which checks that it runs and exits cleanly. Should we add another test which sets up a database with access limited data, runs the script, and then asserts that the database is in the expected state?