Add authorization check on destination post during content import (#668)

The content-import filter already verified the requesting user could
read the source post and validated the nonce, but never checked
whether they were allowed to edit the destination post before writing
imported content into it. Add an explicit current_user_can( 'edit_post',
$dest_post_id ) check alongside the existing checks.

Author: miyanialkesh7

includes/ContentImport/ContentImporter.php

@@ -133,6 +133,10 @@ public function handle_import( array $data = array() ) {
 			return $data;
 		}
 
+		if ( ! current_user_can( 'edit_post', $dest_post_id ) ) {
+			return $data;
+		}
+
 		$import_coordinates = new ImportCoordinates();
 
 		$import_coordinates->source_blog_id = $source_blog_id;