Validate that a dossier on a `for_individual?` procedure always has
an `individual` associated record.
For this, the individual needs to be built before the record is
validated (i.e. even before the `before_create` callback is run).
This should help with #4596: now if a dossier is created without an
`individual`, or if the `invividual` record is later removed, the
validation will fail.
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