Closes #20918

The two factor authentication account string only had the user's
email address. This led to ambiguous entries in two factor
code generating apps. This adds the GitLab host to the account
string in the standard format (according to Google). No matter
the code generator this change disambiguates the entry.

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).

1. This scenario was previously tested for the registration flow, but
   not authentication.

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.

This reverts commit 13e37a3e.

- Also change the help message to indicate that an authenticator app is
  now a prerequisite 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