Cleanup invalid usernames/namespaces
We have a few validations for usernames/namespaces that are not consistent with what we have in the DB - There are usernames that exist on the DB with those words.
We should create a migration and fix this... I couldn't find if we've done that in the past.
More details: https://gitlab.com/gitlab-org/gitlab-ce/issues/29689#note_25718385
/cc @briann @rspeicher