swh-storage: consider preventing bogus permission values

To avoid further occurrences of swh/devel/experiments/swh-db-audit#686 (moved), we should consider whether we need to prevent bogus permissions values in the database.

We need to make sure that there is only a handful of valid values, as wild git repositories can contain weird things.

(the queries to check the data are running)

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Migrated from T687 (view on Phabricator)