Keep attended appointments in calendar events

[jochen-hanisch]

Summary

• keep booked students in scheduler calendar events after attendance has been recorded
• explicitly persist newly created bookings as not attended
• add regression coverage for attended appointments remaining visible in student and teacher calendars

Testing

• php -l classes/model/slot.php
• php -l studentview.controller.php
• php -l tests/slot_test.php
• git diff --check

I could not run the full Moodle PHPUnit suite in this standalone plugin checkout.