rate_limit: Rate limit the /accounts/find/ endpoint.

Closes #19287

This endpoint allows submitting multiple addresses so we need to "weigh"
the rate limit more heavily the more emails are submitted. Clearly e.g.
a request triggering emails to 2 addresses should weigh twice as much as
a request doing that for just 1 address.

zproject/test_extra_settings.py

@@ -265,6 +265,7 @@ RATE_LIMITING_RULES: Dict[str, List[Tuple[int, int]]] = {
     "api_by_remote_server": [],
     "authenticate_by_username": [],
     "create_realm_by_ip": [],
+    "find_account_by_ip": [],
     "password_reset_form_by_email": [],
 }