SimplePie forbit formaction attribute (#7506)

Sanitize buttons with a form or formaction attribute.

lib/lib_rss.php

@@ -348,7 +348,8 @@ function customSimplePie(array $attributes = [], array $curl_options = []): \Sim
 	]);
 	$simplePie->rename_attributes(['id', 'class']);
 	$simplePie->strip_attributes(array_merge($simplePie->strip_attributes, [
-		'autoplay', 'class', 'onload', 'onunload', 'onclick', 'ondblclick', 'onmousedown', 'onmouseup',
+		'autoplay', 'class', 'form', 'formaction',
+		'onload', 'onunload', 'onclick', 'ondblclick', 'onmousedown', 'onmouseup',
 		'onmouseover', 'onmousemove', 'onmouseout', 'onfocus', 'onblur',
 		'onkeypress', 'onkeydown', 'onkeyup', 'onselect', 'onchange', 'seamless', 'sizes', 'srcdoc', 'srcset']));
 	$simplePie->add_attributes([