window.postmessage for Interactive Auth fallback

If you're a webapp running the fallback in an iframe, you can't set set a
window.onAuthDone function. Let's post a message back to window.opener instead.

synapse/rest/client/v2_alpha/auth.py

@@ -77,8 +77,10 @@ SUCCESS_TEMPLATE = """
     user-scalable=no, minimum-scale=1.0, maximum-scale=1.0'>
 <link rel="stylesheet" href="/_matrix/static/client/register/style.css">
 <script>
-if (window.onAuthDone != undefined) {
+if (window.onAuthDone) {
     window.onAuthDone();
+} else if (window.opener && window.opener.postMessage) {
+     window.opener.postMessage("authDone", "*");
 }
 </script>
 </head>