Add option to recover 2FA via SSH

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/3765

Allow users to recover their own account if they lose their 2FA device or recovery codes.

To request new 2FA recovery codes, when you still have access to the SSH key associated with your account:

ssh git@gitlab.com 2fa_recovery_codes

#63 (closed) # Questions/Concerns

• Does this look secure? I think it is but we need to make sure no one can spoof a username or something and disable two factor.

Todo

• Working code
• Tests
• GitLab merge requests (including documentation)