[MM-57067][MM-57006][MM-57328] Acknowledge websocket POSTED events that may result in a notification on the web client (#26604)

* Acknowledge POSTED events that may result in a notification

* Remove temporary metrics

* Add Desktop App support

* Merge'd

* Add some tests

* PR feedback

* Rework window is focused check

* Oops

* Move mentions/followers ACK to posted ACK broadcast hook

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Co-authored-by: Mattermost Build <build@mattermost.com>

server/public/model/websocket_message.go

@@ -92,6 +92,7 @@ const (
 	WebsocketEventChannelBookmarkDeleted                                 = "channel_bookmark_deleted"
 	WebsocketEventChannelBookmarkSorted                                  = "channel_bookmark_sorted"
 	WebsocketPresenceIndicator                        WebsocketEventType = "presence"
+	WebsocketPostedNotifyAck                                             = "posted_notify_ack"
 )
 
 type WebSocketMessage interface {