Allow mappings to be found without knowing the site abbreviation first

- This is useful if a user cannot remember a specific organisation's
  abbreviation but knows the mapping he/she wants to edit. Previously,
  a user had to specify /sites/ABBREVIATION/mappings/PATH. This behaviour
  enables linking to a specific mapping from external applications and
  sites, i.e. the side-by-side browser.
- Add test cases for this behaviour.
Fix some bad indentation at the end of MappingsController#find_global
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.
Render an error message if a URI::InvalidURIError exception occurs
Merge pull request #191 from alphagov/find_mappings_from_full_url

Allow mappings to be found without knowing the site abbreviation