Establish a standardized serialization format for u2f registration
keys: base64-encoded raw registration data, encoded as a JSON
string. This format decodes transparently to a Go []byte slice, but it
needs explicit base64 decoding in Python.

Previously it was deleting aliases instead of adding them.

The type is already encoded in the resource ID, but this is handy for
API clients so that they do not need to parse resource IDs.

Make it possible for the sso-server to fetch the recovery hint using
the same API.

It makes more sense to have this directly on the User object.

Adds UIDs to users, websites, DAV accounts.

Assign a random UID to newly created users, and ensure that all
associated resources have the same UID as well.

Check passwords and recovery responses directly against backend data.

Still doesn't entirely solve the problem of encoding the U2F
registration requests on the client side (we should probably just
accept a bytestring), but all the important functionality is there.

Also set an initial user password, and random passwords for all its
resources, on a CreateUser request.