Make U2F Check based on actual window.u2f availability rather than specific browser versions

The current U2F interface for registering U2F devices uses a whitelist of specific browsers and versions.

I currently use a U2F device in Safari for services using this browser extension from Ybico:

https://github.com/Safari-FIDO-U2F/Safari-FIDO-U2F

It adds the window.u2f object to fully support U2F devices for 2FA.

It'd be great for the registration page to check for the availability of this object, rather than rely on a hardcoded list of browser version, that way as more browsers implement it, or if people want to use extensions, it'll automatically just work.