Cleanup invalid usernames/namespaces

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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