WebSearch: journal hint service utf8 fix

* Fixes journal hint service: decodes utf8 before passing the query
  to `search_from_reference`

* Adds regression test to journak hint service for the utf8 query

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

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