user_groups: Add server level setting disallow anonymous groups for settings.

This commit adds a server level setting which controls whether the setting
can be set to anonymous user groups. We only allow it in the tests for
now because the UI can only handle named user groups.

zerver/lib/user_groups.py

@@ -2,6 +2,7 @@ from contextlib import contextmanager
 from dataclasses import dataclass
 from typing import Collection, Dict, Iterable, Iterator, List, Mapping, Optional, TypedDict, Union
 
+from django.conf import settings
 from django.db import connection, transaction
 from django.db.models import F, Prefetch, QuerySet
 from django.utils.timezone import now as timezone_now
@@ -712,6 +713,7 @@ def get_server_supported_permission_settings() -> ServerSupportedPermissionSetti
 
 def parse_group_setting_value(
     setting_value: Union[int, AnonymousSettingGroupDict],
+    setting_name: str,
 ) -> Union[int, AnonymousSettingGroupDict]:
     if isinstance(setting_value, int):
         return setting_value
@@ -719,6 +721,13 @@ def parse_group_setting_value(
     if len(setting_value.direct_members) == 0 and len(setting_value.direct_subgroups) == 1:
         return setting_value.direct_subgroups[0]
 
+    if not settings.ALLOW_ANONYMOUS_GROUP_VALUED_SETTINGS:
+        raise JsonableError(
+            _("{setting_name} can only be set to a single named user group.").format(
+                setting_name=setting_name
+            )
+        )
+
     return setting_value