Confirmation email + Subscription pages

- form as an iframe: increased marginY
- fixed issue with page titles (old themes using wp_title hook)
- redirect on reinstall instead of showing a message (to avoid being able to re-save old settings)

assets/js/src/iframe.js

@@ -1,7 +1,7 @@
 define('iframe', ['mailpoet', 'jquery'], function(MailPoet, jQuery) {
   'use strict';
   MailPoet.Iframe = {
-    marginY: 15,
+    marginY: 20,
     autoSize: function(iframe) {
       if(!iframe) return;