Restrict failed login attempts for users with 2FA

What does this MR do?

Restrict failed login attempts from users with 2FA enabled.

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

This code is mostly copied from Devise::Models::Lockable#valid_for_authentication?, as our custom login flow with two pages doesn't call this method.

Why was this MR needed?

We only lock accounts without 2FA enabled when they fail to successfully log in enough times in a row.

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/19799.