Integrate with the API for adding and getting webauthn creds

This links up the `get_webauthn_credentials_for_user` and
`create_webauthn_credential_for_user` methods of the user api client to
notifications-api.

To send data to the API we need strings to be unicode, so we call
decode('utf-8') on base64 objects.

Co-authored-by: Leo Hemsted <leo.hemsted@digital.cabinet-office.gov.uk>

app/notify_client/user_api_client.py

@@ -192,19 +192,14 @@ class UserApiClient(NotifyAdminAPIClient):
         return self.get(endpoint)
 
     def get_webauthn_credentials_for_user(self, user_id):
-        # TODO: remove when using real API
-        self.credentials = getattr(self, 'credentials', [])
-        return self.credentials
+        endpoint = f'/user/{user_id}/webauthn'
+
+        return self.get(endpoint)['data']
 
     def create_webauthn_credential_for_user(self, user_id, credential):
-        self.credentials = getattr(self, 'credentials', [])
-        credential_dict = credential.serialize()
+        endpoint = f'/user/{user_id}/webauthn'
 
-        # TODO: remove when using real API
-        from datetime import datetime
-        credential_dict['created_at'] = datetime.now().strftime("%Y-%m-%dT%H:%M:%S.%fZ")
-
-        self.credentials += [credential_dict]
+        return self.post(endpoint, data=credential.serialize())
 
 
 user_api_client = UserApiClient()