3266584: avoid duplicates during import

3266584: added the option to reject duplicate reference entries based on items using existing DOI, PMID, or PMCID values.

This adds a single custom entity-level constraint that checks uniqueness among the bibcite_doi, bibcite_pmid, and bibcite_pmcid fields. The check cascades; if it finds a duplicate DOI entry, it does not continue to check PMID and PMCID, and so on.

This behavior is configurable on the Import admin page (/admin/config/bibcite/settings/import), and is off by default. There is an update hook that adds the configuration to existing websites.

Closes #3266584