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
Instead of deleting a namespace synchronously as before, we now mark the namespace for deletion, disable it, and rename it. A worker then comes along and deletes the namespace in the background. This results in a *significantly* better user experience, as the namespace deletion operation now "completes" in under a second, where before it could take 10s of minutes at the worse.
Fixes https://jira.coreos.com/browse/QUAY-838
