Fix unlist button and add reasons to email

[MaggieCabrera]

Summary

The Unlist button and its modal were not showing up on production. The editor plugin imports PluginPostStatusInfo and PluginDocumentSettingPanel from @wordpress/edit-post, but those were moved to @wordpress/editor in newer WordPress. On versions where the old location no longer provides them, the import is empty and the whole pattern editor plugin fails to render, so the button, the modal, and the Pattern Details panel all disappear at once. We now import from both packages and use whichever the running version provides, so it works on old and new WordPress.

While fixing this, we also made the modal's textarea actually useful to the author. Before, that field was saved as an internal note that only moderators could see, even though moderators often want to tell the author what is wrong so they can fix it and resubmit. Now the text is included in the unlisted email that goes to the author. We renamed the field to "Message to the pattern author" so it is clear the content will be sent to them.

How to test

1. Open a pattern in the editor and confirm the Unlist button appears in the Status and visibility panel.
2. Click Unlist, pick a reason, and write a message in the textarea.
3. Submit and confirm the pattern is unlisted and the author receives an email containing both the reason and your message.

This was working locally before my fix, and I don't know if I can reproduce the failure locally with the environment from this repo, so this fix is done a bit in the dark.