1. Display a list of U2F devices on the `two_factor_auth` page.
2. Allow deleting individual U2F devices.
3. Allow setting a (optional) name for a device (during registration).
Allow U2F devices to be used in Firefox
- Adds U2F support for Firefox
- Improve U2F feature detection logic
- Have authentication flow be closer to the spec (single challenge instead of a challenge for each `signRequest`)
- Closes#17341
- Related to #15337
See merge request !5177
1. Only on supported Chrome versions
2. Mainly, this lets us simplify the javascript-based U2F check to
`window.u2f`, where `window.u2f` can either be loaded from the GitLab
server (for Chrome) or from the Firefox extension.
3. This is a better way to provide browser detection for U2F.
- Move the `TwoFactorAuthsController`'s `new` action to `show`, since
the page is not used to create a single "two factor auth" anymore. We
can have a single 2FA authenticator app, along with any number of U2F
devices, in any combination, so the page will be accessed after the
first "two factor auth" is created.
- Add the `u2f` javascript library, which provides an API to the
browser's U2F implementation.
- Add tests for the JS components
