WebSearch: new service JournalHintService

* Adds the search service JournalHint, that displays "Were you looking for a journal reference? Try: [link]" when the user search for a raw journal reference.
* Adds regression tests to WebSearchServicesJournalHintService.
* Uses refextract to detect if the query is a raw journal reference

Signed-off-by: Federico Poli <federico.poli@cern.ch>
1: D400 First line should end with '.', not 'y'
1: D205 Blank line missing between one-line summary and description
34: D203 Expected 1 blank line *before* class docstring, found 0
61: D205 Blank line missing between one-line summary and description