Rake task to erase 2FA data for all users
Original issue: https://dev.gitlab.org/gitlab/gitlabhq/issues/2435
In a 'disaster' scenario it can happen that a GitLab installation loses the secret key that encrypts all 2FA bar codes in the database. When that happens users who set up 2FA can no longer log in. We should have a rake task that clears all 2FA bar codes to allow GitLab admins to deal with this scenario.