Sanitize and sandbox toot embeds (#9552)

app/javascript/mastodon/features/ui/components/embed_modal.js

@@ -77,6 +77,7 @@ class EmbedModal extends ImmutablePureComponent {
             className='embed-modal__iframe'
             frameBorder='0'
             ref={this.setIframeRef}
+            sandbox='allow-same-origin'
             title='preview'
           />
         </div>