FIX: prevents PM to large groups (#28681)

This commit introduces a new hidden site setting: `group_pm_user_limit`, default to `1000` which will raise an error when attempting to create a PM target a large group.

lib/topic_creator.rb

@@ -109,7 +109,18 @@ class TopicCreator
     end
 
     topic.reload.topic_allowed_groups.each do |topic_allowed_group|
-      topic_allowed_group.group.set_message_default_notification_levels!(topic)
+      group = topic_allowed_group.group
+
+      begin
+        group.set_message_default_notification_levels!(topic)
+      rescue Group::GroupPmUserLimitExceededError => e
+        rollback_with!(
+          topic,
+          :too_large_group,
+          group_name: group.name,
+          limit: SiteSetting.group_pm_user_limit,
+        )
+      end
     end
   end