This change ensures that the tables in the database during migration have at least one row of "real" data, which should help catch issues in the future where we forget to set column defaults and other such schema oversights that can only be truly tested with non-empty tables
Fixes https://jira.coreos.com/browse/QUAY-913
The checksum field was removed from the ImageStorage model in #815,
but was never dropped from the database. This adds a migration to
drop the unused column.