Slugify the `redirect_from` URLs

This ensures that URLs posted in a `redirect_from` are slugified. Note that the regex method is slightly different than the one [from Rails](https://github.com/rails/rails/blob/08754f12e65a9ec79633a605e986d0f1ffa4b251/activesupport/lib/active_support/inflector/transliterate.rb#L85-L93) because we need to support the `/` here. 

This would allow people to refer to redirect articles by their page title, rather than slug. I'm all about letting humans use human words, `not-constructing-text-that-looks-like-this`. 
We can't use `compact` here because it messes with the `has_redirect_to_url?` check below.