user_groups: Add transaction.atomic decorator to add_user_group view.

The database operations in 'access_user_group_for_setting' and
'check_add_user_group' used in 'add_user_group' view should be
collectively atomic.

This commit adds transaction.atomic decorator for that purpose.

zerver/actions/user_groups.py

@@ -26,7 +26,7 @@ from zerver.models import (
 )
 from zerver.models.groups import SystemGroups
 from zerver.models.users import active_user_ids
-from zerver.tornado.django_api import send_event, send_event_on_commit
+from zerver.tornado.django_api import send_event_on_commit
 
 
 class MemberGroupUserDict(TypedDict):
@@ -35,7 +35,7 @@ class MemberGroupUserDict(TypedDict):
     date_joined: datetime
 
 
-@transaction.atomic
+@transaction.atomic(savepoint=False)
 def create_user_group_in_database(
     name: str,
     members: list[UserProfile],
@@ -182,7 +182,7 @@ def do_send_create_user_group_event(
             can_mention_group=get_group_setting_value_for_api(user_group.can_mention_group),
         ),
     )
-    send_event(user_group.realm, event, active_user_ids(user_group.realm_id))
+    send_event_on_commit(user_group.realm, event, active_user_ids(user_group.realm_id))
 
 
 def check_add_user_group(

zerver/views/user_groups.py

@@ -45,6 +45,7 @@ from zerver.models.users import get_system_bot
 from zerver.views.streams import compose_views
 
 
+@transaction.atomic(durable=True)
 @require_user_group_edit_permission
 @typed_endpoint
 def add_user_group(