archiver: Handle forced copies of contents not registered in the archiver database

The archiver database is lagging behind the actual database (ref. T494).

The forced "copy to backend" worker needs to cope with that.

Suggestion: if a content doesn't exist in the archiver database, check whether it exists on a given source backend (new config option), and if it does, populate the archiver database to record so.

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Migrated from T569 (view on Phabricator)