Show intro only on new install (but not when migrating from WP2)

Intro is marked as shown only when "Skip" or "Done" is explicitly used - this
avoids closing the intro by mistake and never having the option to see it again.

[MAILPOET-1446]

lib/Config/Changelog.php

@@ -47,6 +47,7 @@ class Changelog {
         } else {
           $skip_wizard = WPHooks::applyFilters('mailpoet_skip_welcome_wizard', false);
           $redirect_url = $skip_wizard ? null : admin_url('admin.php?page=mailpoet-welcome-wizard');
+          Setting::setValue('show_intro', true);
         }
       }
     }