Set a uniqueness constraint on the content generator name in the
database.
Prior to this change, we were only enforcing this in the hub application
layer. Configure this in postgres for safety.
Changing owner is triggering repo regeneration in kojira. As it is
something which doesn't have any effect on repodata, let's put it to
separate table and let kojira ignore these changes.
Fixes: https://pagure.io/koji/issue/956
