WebSearch: new Journal Hint Service

* Adds the Journal Hint Service, that displays
  'Were you looking for a journal reference? Try: <link>'
  when the user search for a raw journal reference.

* Adds unit and regression tests for the new search service.

* Adds two unit tests to `search_from_reference`, just to be sure that
  it accepts unicode

Signed-off-by: Federico Poli <federico.poli@cern.ch>