Fix not showing a message that instructs users to NOT use an image larger than 5MB for Twitter

[leonidasmi]

In packages\helpers\src\social-preview-image-validation\twitterValidation.js we have the validatesBytes() function that's supposed to show a The file size of the uploaded image is too large for X. File size must be less than 5MB message to users that try to use a large image as Twitter image.

That function is not used properly, because that message is never shown.

We have to fix this, so that users can be instructed not to use such big images, as it violates Twitter's guidelines

(Note, you probably have to use add_filter( 'big_image_size_threshold', '__return_false' ); to be able to upload such big images)