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/post_test.go

@@ -1593,12 +1593,12 @@ func TestUpdatePost(t *testing.T) {
 
 		testPost, err = th.App.CreatePost(th.Context, testPost, channelForTestPost, false, false)
 		require.Nil(t, err)
-		assert.Equal(t, testPost.GetProps(), model.StringInterface{})
+		assert.Equal(t, model.StringInterface{}, testPost.GetProps())
 
 		testPost.Message = permalink
 		testPost, err = th.App.UpdatePost(th.Context, testPost, false)
 		require.Nil(t, err)
-		assert.Equal(t, testPost.GetProps(), model.StringInterface{"previewed_post": referencedPost.Id})
+		assert.Equal(t, model.StringInterface{model.PostPropsPreviewedPost: referencedPost.Id}, testPost.GetProps())
 	})
 
 	t.Run("sanitizes post metadata appropriately", func(t *testing.T) {