Blacklist existing keys to avoid re-registering a key under the same user

I saw a TODO that mentioned blacklisting existing keys, and implemented it. This involved creating a SignRequest for each key that a user had already registered, and passing them as a context variable so they could be given to the JavaScript client API (as the second parameter of `u2f.register`).

Now, in testproj, attempting to add a key already in use by the given user will result in an error code of 4, [corresponding to "DEVICE_INELIGIBLE"](http://fidoalliance.org/specs/fido-u2f-v1.0-ps-20141009/fido-u2f-javascript-api-ps-20141009.html#error-codes) (which is the expected behavior).

Note that this doesn't actually blacklist keys by itself; if someone uses django-u2f and doesn't use the `sign_requests` context variable as part of their call to `u2f.register`, the key will get registered again. It's up to them to use it properly (as in testproj).