feat: Add a 403 and 429 handler.

See context here: https://django-ratelimit.readthedocs.io/en/latest/cookbook/429.html#context

For now we continue to fall back to django's default 403 handler for 403
but provide a new 429 template that we use for ratelimit exceptions.

This commit also updates a logistration test that relied on the old 403
behavior of django-ratelimit instead of the newly added 429 behavior.

openedx/core/djangoapps/user_authn/views/tests/test_logistration.py

@@ -137,7 +137,7 @@ class LoginAndRegistrationTest(ThirdPartyAuthTestMixin, UrlResetMixin, ModuleSto
 
         # then the rate limiter should kick in and give a HttpForbidden response
         response = self.client.get(login_url)
-        self.assertEqual(response.status_code, 403)
+        self.assertEqual(response.status_code, 429)
 
         # now reset the time to 6 mins from now in future in order to unblock
         reset_time = datetime.now(UTC) + timedelta(seconds=361)