Remove the "Start upgrade" button when the upgrade is completed

Add the "Upgrade completed" message
Add the "Go to MailPoet" button

lib/Config/MP2Migrator.php

@@ -145,6 +145,7 @@ class MP2Migrator {
 
     Setting::setValue('mailpoet_migration_complete', true);
 
+    $this->log("IMPORT COMPLETE");
     $this->log(sprintf('=== ' . __('END IMPORT', 'mailpoet') . ' %s ===', $datetime->formatTime(time(), \MailPoet\WP\DateTime::DEFAULT_DATE_TIME_FORMAT)));
     $result = ob_get_contents();
     ob_clean();