Rectify some of the issues raised in #191

- Refactor aka -> www substitutions into the Host model.
- Change aka_url variable to url, because it won't always be an aka
  URL being passed in. Change occurrences of uri to url for consistency.
- Don't interpolate url.host in the `where hostname` clause.
- Change "url not present" 404 to 400, bad request, HTTP status code.