OpenID Connect RP support for customer accounts

src/pretix/static/pretixpresale/js/postmessage.js

@@ -0,0 +1,10 @@
+/*global $ */
+
+$(function () {
+    window.addEventListener("message", (event) => {
+        if (event.data && event.data.__process === "popup_close") {
+            window.close()
+        }
+    });
+    window.opener.postMessage(JSON.parse($("#postmessage").text()), $("#origin").text())
+})