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.