WIP: prevent users from removing their last authentication method

username-removed-226882 requested to merge BM5k/gitlab-ce:issues/3332 into master

What does this MR do?

Ensure users always have at least one way to log in

Are there points in the code the reviewer needs to double check?

I have no idea where/how to test this

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

What are the relevant issue numbers?

#3332 (moved)

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