restructure bounce app

www/bounce/main.js

@@ -1,20 +1,57 @@
 define(['/api/config'], function (ApiConfig) {
+    var reject = function () {
+        window.close();
+    };
     if (ApiConfig.httpSafeOrigin !== window.location.origin) {
         window.alert('The bounce application must only be used from the sandbox domain, ' +
             'please report this issue on https://github.com/xwiki-labs/cryptpad');
         return;
     }
-    var bounceTo = decodeURIComponent(window.location.hash.slice(1));
-    if (!bounceTo) {
-        window.alert('The bounce application must only be used with a valid href to visit');
-        return;
-    }
-    if (bounceTo.indexOf('javascript:') === 0 || // jshint ignore:line
-        bounceTo.indexOf('vbscript:') === 0 || // jshint ignore:line
-        bounceTo.indexOf('data:') === 0) {
-        window.alert('Illegal bounce URL');
-        return;
+    if (typeof(URL) !== 'function') {
+        window.alert("Your browser does not support functionality this page requires");
+        return void reject();
     }
+
     window.opener = null;
-    window.location.href = bounceTo;
+
+    var host;
+    try {
+        host = new URL('', ApiConfig.httpUnsafeOrigin);
+    } catch (err) {
+        window.alert("This server is configured incorrectly. Its administrator should check its diagnostics page");
+        return void reject();
+    }
+
+    var target;
+    try {
+        var bounceTo = decodeURIComponent(window.location.hash.slice(1));
+        target = new URL(bounceTo, ApiConfig.httpUnsafeOrigin);
+    } catch (err) {
+        console.error(err);
+        window.alert('The bounce application must only be used with a valid href to visit');
+        return void reject();
+    }
+
+    var go = function () {
+        window.location.href = target.href;
+    };
+
+    if (target.host === host.host) { return void go(); }
+
+    require([
+        '/customize/messages.js',
+    ], function (Messages) {
+        Messages.bounce_confirm = 'You are about to leave {0}\n\nAre you sure you want to visit "{1}"?'; // XXX
+        Messages.bounce_danger = 'It looks like someone is trying to trick you into visiting a dangerous link.\n\n("{0}")\n\nBe careful!'; // XXX
+
+        if (['javascript:', 'vbscript:', 'data:', 'blob:'].includes(target.protocol)) {
+            window.alert(Messages._getKey('bounce_danger', [target.href]));
+            return void reject();
+        }
+
+        var question = Messages._getKey('bounce_confirm', [host.hostname, target.href]);
+        var answer = window.confirm(question);
+        if (answer) { return void go(); }
+        reject();
+    });
 });