remove course notifications as well when deleting user and cleanup table via migration, fixes #2141

Closes #2141

Merge request studip/studip!1389

db/migrations/5.1.41_cleanup_seminar_user_notifications.php

+<?php
+
+/**
+ * @see https://gitlab.studip.de/studip/studip/-/issues/2141
+ */
+final class CleanupSeminarUserNotifications extends Migration
+{
+    public function description()
+    {
+        return "Removes all entries from `seminar_user_notifications` that are not in `seminar_user` as well";
+    }
+
+    protected function up()
+    {
+        $query = "DELETE `seminar_user_notifications`
+                          FROM `seminar_user_notifications`
+                          LEFT JOIN `seminar_user` USING(`user_id`, `Seminar_id`)
+                          WHERE `seminar_user`.`user_id` IS NULL";
+        DBManager::get()->exec($query);
+    }
+}