Add support for FIDO U2F

FIDO U2F is a secure and convenient method of two factor authentication.

GitHub already supports this, as does Google logins.

One of the most widely used security tokens, the Yubikey supports this.

Specifications can be downloaded here: https://fidoalliance.org/specifications/download/

FIDO U2F security keys are now supported in Bitbucket. Visit two-step verification settings to add your key. If you do not already have two-step verification enabled, you’ll need to enable it before you can use your U2F key with Bitbucket.

If you have any questions or issues, please comment on this issue. We’ll update the 2SV documentation in the next few days to include this feature. Thanks!