MM-42810: Using websocket broadcast hook for permalink preview (#28627)

We use the newly introduced websocket broadcast hook system
to implement permalink preview efficiently. This is essentially
a re-do of https://github.com/mattermost/mattermost/pull/23812
using the new system.

https://mattermost.atlassian.net/browse/MM-42810
```release-note
NONE
```

---------

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

server/public/model/notification.go

@@ -22,6 +22,7 @@ const (
 
 	NotificationReasonFetchError                         NotificationReason = "fetch_error"
 	NotificationReasonParseError                         NotificationReason = "json_parse_error"
+	NotificationReasonMarshalError                       NotificationReason = "json_marshal_error"
 	NotificationReasonPushProxyError                     NotificationReason = "push_proxy_error"
 	NotificationReasonPushProxySendError                 NotificationReason = "push_proxy_send_error"
 	NotificationReasonPushProxyRemoveDevice              NotificationReason = "push_proxy_remove_device"

server/public/model/post.go

@@ -878,8 +878,11 @@ func (o *Post) ForPlugin() *Post {
 }
 
 func (o *Post) GetPreviewPost() *PreviewPost {
+	if o.Metadata == nil {
+		return nil
+	}
 	for _, embed := range o.Metadata.Embeds {
-		if embed.Type == PostEmbedPermalink {
+		if embed != nil && embed.Type == PostEmbedPermalink {
 			if previewPost, ok := embed.Data.(*PreviewPost); ok {
 				return previewPost
 			}