Destroy filter tagging when filter (tag) is destroyed

Check that filter isn't nil before attempting to expire the cache on it
Correct wording in draft purge rake task
Refactoring to new active record syntax

Also, it's more informative to show an "i out of n" counter when going through existing taggings. Though I don't know if we'll use that method again on all our millions (of filter taggings). *g*
Merge branch 'refs/heads/master' into issue-3939-filter_taggings
Taking out the dependent destroy bit, because not sure it's the right thing and is not connected to the bug at hand
Merge pull request #1577 from CristinaRO/issue-3939-filter_taggings

Issue 3939 filter taggings