Add banner for broadcast messages

Hello Zabbix Team !

Just a small feature request : a banner with a broadcast message can be used to advertise users for a maintenance, important information (like update, reboot), etc.

I like the banner of Gitlab for example : https://docs.gitlab.com/ee/user/admin_area/broadcast_messages.html

Maybe we can use the native warning section like this :

Thank you for your work!