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/channels/app/notification_test.go

@@ -413,7 +413,7 @@ func TestSendNotifications_MentionsFollowers(t *testing.T) {
 					{
 						Type: model.PostEmbedPermalink,
 						URL:  postURL,
-						Data: &model.Post{},
+						Data: &model.PreviewPost{},
 					},
 				},
 			},