feat: add open questions with a pending grading state

- migration: open question type, rubric, max_length, quiz grader and
  the grading and review columns of attempt answers
- open answers are checked, stored as pending and never scored on
  answer; a changed text resets the grading state
- student JSON gains text, grading_status, grader_feedback,
  review_comment, max_length, needs_review and open_answers; the
  rubric is never serialized

plugin_test.go

@@ -43,8 +43,8 @@ func TestPluginActivates(t *testing.T) {
 	if len(migrations) == 0 {
 		t.Fatal("no migrations")
 	}
-	if got := migrations[len(migrations)-1].ID; got != "20261109120000_add_test_mode_to_quizzes" {
-		t.Errorf("last migration = %s, want 20261109120000_add_test_mode_to_quizzes", got)
+	if got := migrations[len(migrations)-1].ID; got != "20261109180000_add_open_questions_and_grading" {
+		t.Errorf("last migration = %s, want 20261109180000_add_open_questions_and_grading", got)
 	}
 	if app.Config.Bool("golem15.quizzes.scoring.multiple_partial") {
 		t.Error("scoring.multiple_partial defaults to true, want false")