To avoid having to implement legacy code handling for the obsolete
`verified_signature` attribute and to avoid any race conditions during
the zero-downtime-deployment we do the following:

1. Destroy all records
2. Migration: Use add_column_with_default to add the new attribute and
   update the verification_status values on records created between 1.
   and 2.
3. Deploy the new code
4. Post migration: Destroy all records

Like this we make sure that at no point there is a record with a `nil`
value for the new `verification_status` attribute.