Avoid registering the same authenticator twice

This passes existing credentials in the server response, to allow
the browser to prevent re-registering the same key for the same
user. Registering the same key multiple times doesn't seem to be
an issue technically; the user has likely got their keys mixed up.

- Chrome says "you don't need to register it again".
- Safari exits with an InvalidStateError.
- Firefox exits with a DOMException.

tests/conftest.py

@@ -4485,7 +4485,7 @@ def mock_get_invited_org_user_by_id(mocker, sample_org_invite):
 def webauthn_credential():
     return {
         'name': 'Test credential',
-        'credential_data': 'credential_data',
+        'credential_data': b'WJ0AAAAAAAAAAAAAAAAAAAAAAECKU1ppjl9gmhHWyDkgHsUvZmhr6oF3/lD3llzLE2SaOSgOGIsIuAQqgp8JQSUu3r/oOaP8RS44dlQjrH+ALfYtpAECAyYhWCAxnqAfESXOYjKUc2WACuXZ3ch0JHxV0VFrrTyjyjIHXCJYIFnx8H87L4bApR4M+hPcV+fHehEOeW+KCyd0H+WGY8s6',  # noqa
         'registration_response': 'anything',
         'created_at': '2017-10-18T16:57:14.154185Z',
     }