Use a single challenge for U2F authentication.

1. According to the spec, either we have a single challenge with
   a number of `signRequests`, or a number of `signRequests`, each with
   it's own challenge.

2. Previously, we had both these - per-request challenges, as well as a
   single extra challenge.

3. This commit changes this so that the per-request challenges are
   removed, leaving only a single challenge, as per the v1.1 U2F API.

4. The existing implementation didn't work in Firefox, because the
   Firefox (extension) implementation is less flexible with regard to
   the inputs.

5. Fix teaspoon specs.

6. References: https://fidoalliance.org/specs/fido-u2f-v1.0-nfc-bt-amendment-20150514/fido-u2f-javascript-api.html#h2_background