Newsletter: Add support for enableing or disabeling of sending newsletter in the prepublish modal

[enejb]

Expected behaviour

Currently there is no way to disable or enable the sending of newsletters before publishing a post from the app.

Actual behavior

Currently the pre-publish modal looks like this.

It would be great if we could add a toggle like this. That currently appears before you publish a blog post.

[dangermattic]

Thanks for reporting! 👍