make verifyClaimConcurrency also check Wikidata for non-ISBN external ids

[maxlath]

verifyClaimConcurrency currently only checks the local entities database, as it was initially designed for ISBNs (properties for which we currently ignore Wikidata, cf #182).

I had the case that I re-added a Theses.fr person (wdt:P4285) ID (087735768) to a local entity, while there was already a duplicate author with that ID on Wikidata (wd:Q45463524). In that case, we could either

• refuse adding the ID to the local entity (as is the case today when trying to use an ISBN already used), and open a deduplicate task (but with the issue that this could happen for entities of any type, which do not support at the moment)
• refuse it but offer to merge with the concurrent entity, once all users can attempt to merge entities allow non-dataadmins users to merge entities #491
• accept it and trigger a /api/tasks?action=check-entities on the local entity, as that should then trigger an automerge, based on the external ids checks (again with the limitation that this could happen for entities of any type, which tasks do not support at the moment)