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>
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