Block Editor: Add preloading of autosaves.

With the introduction of WordPress/gutenberg#7945, the block editor requests autosave data when the editor is loaded. This can be optimized by preloading the request server-side and then passing the request data to the client using the preloading mechanism in editor-form-blocks.php.

Props: talldanwp, aduth.

git-svn-id: http://develop.svn.wordpress.org/trunk@46110 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/edit-form-blocks.php

@@ -49,6 +49,7 @@
 	sprintf( '/wp/v2/users/me?post_type=%s&context=edit', $post_type ),
 	array( '/wp/v2/media', 'OPTIONS' ),
 	array( '/wp/v2/blocks', 'OPTIONS' ),
+	sprintf( '/wp/v2/%s/%d/autosaves?context=edit', $rest_base, $post->ID ),
 );
 
 /**