When the grants administrator recieves all recipients, invoices and other documents required for reimbursement process, they make the ticket with "expense document filled" ack. Ordinary users shoudn't be able to delete expense documents after that, because that allows them to delete evidence.
Create a permission that allows to control who is able to delete docs for deleted tickets.
Student is expected to send a patch for wikimedia-cz/tracker repository, hosted at Wikimedia Gerrit. When claiming task on GCI website, students should claim a respective Phabricator task as well.