[meta] Allow 2FA Login Using a U2F Device

• Use a U2F device as an alternative to authenticator apps.
• Best example of a U2F device is Yubico's FIDO U2F Security Key
• https://www.yubico.com/
• https://www.yubico.com/products/yubikey-hardware/

Suggested in #2979 (comment 4868554) /cc @JobV @matt.wilkinson

Features

This serves as an umbrella issue for all U2F features:

• Register U2F device and authenticate with it (!3905 (merged))
• Require authenticator to be set up before enabling U2F (#17333 (closed))
• Support for Firefox via extension (#17341 (closed))
• Add an identifier for each U2F device (#17334 (closed))
• More granular control over disabling U2F devices (#17335 (closed))
• Honor the "Remember Me" parameter (#18103 (closed))
• Polish up the U2F flow (#18556 (closed))
• Register an U2F device should trigger an email and an audit log event (#18557 (moved))
• Support for more browsers (#22938 (moved) and gitlab-org/gitlab-ee#778)

TODO After Core Implementation

• Test with other vendors' U2F devices

  • Order non-yubico U2F devices

  • Test with non-yubico devices

    • I've tested our U2F implementation with these three devices, with no issues:
      • Yubico FIDO U2F: https://www.amazon.com/dp/B00NLKA0D8
      • Plug-Up Security Key: https://www.amazon.com/dp/B00OGPO3ZS/
      • HyperFIDO Security Key: https://www.amazon.com/dp/B00WIX4JMC

• Audit ruby-u2f gem and u2f javascript API