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
This could result in "hanging" robot accounts, although that would only leak the names of said accounts. Now we delete them immediately AND we proactively delete them before replacing the namespace (just to be sure)