invoices

• Fix warning found into /var/log/error.log when using PHP 8

• Create / Add phpunit tests

At the moment no events will be logged.
Events must be logged and written to the DB. The entity Log shall be used for this.

In another ticket these events will be displayed in the web page and can be reviewed by the admin user.

Every event that triggers a create, update or delete of an entity should be logged.
User supplied input must be sanitized before writing to the log.

Add counter that will log who has downloaded the invoice and how many times the invoice has been downloaded.