PERF: Make chat mention notifications async. (#19666)

This PR removes the limit added to max_users_notified_per_group_mention during #19034 and improve the performance when expanding mentions for large channel or groups by removing some N+1 queries and making the whole process async.

* Fully async chat message notifications

* Remove mention setting limit and get rid of N+1 queries

plugins/chat/app/jobs/regular/send_message_notifications.rb

@@ -0,0 +1,21 @@
+# frozen_string_literal: true
+
+module Jobs
+  class SendMessageNotifications < ::Jobs::Base
+    def execute(args)
+      reason = args[:reason]
+      valid_reasons = %w[new edit]
+      return unless valid_reasons.include?(reason)
+
+      return if (timestamp = args[:timestamp]).blank?
+
+      return if (message = ChatMessage.find_by(id: args[:chat_message_id])).nil?
+
+      if reason == "new"
+        Chat::ChatNotifier.new(message, timestamp).notify_new
+      elsif reason == "edit"
+        Chat::ChatNotifier.new(message, timestamp).notify_edit
+      end
+    end
+  end
+end