fix warnings on PHP 8, re #160

controllers/config.php

controllers/groups.php

controllers/sheets.php

exercises/Exercise.php

exercises/cloze_exercise.php

exercises/lt_exercise.php

exercises/seq_exercise.php

exercises/tb_exercise.php

lib/VipsAssignment.php

views/exercises/cloze_exercise/correct.php

views/exercises/cloze_exercise/print.php

views/exercises/cloze_exercise/xml.php

views/exercises/correct_exercise.php

views/exercises/lt_exercise/xml.php

views/exercises/sc_exercise/xml.php

views/exercises/seq_exercise/xml.php

views/exercises/tb_exercise/correct.php

views/exercises/tb_exercise/solve.php

views/exercises/tb_exercise/xml.php

views/sheets/copy_assignment_dialog.php