Backwards compatibility with traditional admin notices

[mboynes]

Following #4811, if a plugin or theme is trying to show a message on the post edit screen, it will be hidden. Plugins may be trying to add important messages, ones that only appear on the new/edit post screen. Gutenberg should attempt to render these messages, perhaps by automatically upgrading them to notice components, rather than hiding them.

To Reproduce
Steps to reproduce the behavior:

1. Render an admin notice, e.g.

   add_action(
   	'admin_notices',
   	function() {
   		echo '<div class="updated success"><p>Sample Message</p></div>';
   	}
   );

2. Navigate to the new/edit posts screen with Gutenberg active

Expected behavior

I should see the above "Sample Message", ideally with some indicator of success.

Additional context

Related: #3395, amongst others.

[danielbachhuber]

#6388 is the canonical issue for this. Closing in favor of that.

[collimarco]

This still doesn't work... admin notices are not shown in Gutenberg.

Moreover the linked issue has been closed as completed, but the bug is still present.

Please fix this, so that admin_notices works as expected...