WebSearch: journal hint service utf8 fix

* Fixes journal hint service: decodes utf8 before passing the query
  to `search_from_reference`

* 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>
`p` is an `utf8` string, an array of bytes, right? Apparently docextract requires an unicode object, so that line converts `p` from `utf8` to `unicode`