remote_billing: Add rate-limiting for confirmation email sending.

These should be rate-limited by both IP using our regular
sends_email_by_ip bucket as well as by server, using a new bucket
dedicated to this.

zproject/default_settings.py

@@ -291,6 +291,12 @@ DEFAULT_RATE_LIMITING_RULES = {
         # 1000 per day per file
         (86400, 1000),
     ],
+    # A zilencer-only limit that applies to requests to the
+    # remote billing system that trigger the sending of an email.
+    "sends_email_by_remote_server": [
+        # 10 emails per day
+        (86400, 10),
+    ],
 }
 # Rate limiting defaults can be individually overridden by adding
 # entries in this object, which is merged with

zproject/test_extra_settings.py

@@ -256,6 +256,7 @@ RATE_LIMITING_RULES: Dict[str, List[Tuple[int, int]]] = {
     "sends_email_by_ip": [],
     "email_change_by_user": [],
     "password_reset_form_by_email": [],
+    "sends_email_by_remote_server": [],
 }
 
 CLOUD_FREE_TRIAL_DAYS: Optional[int] = None