feat: add test mode to quizzes and withhold its solutions

- mode, navigation and max_attempts columns with Go-side defaults
- classes.Withheld gates every solution field of the answer feedback
  and the attempt result while a test attempt is open
- a test's quiz detail lists no questions before the attempt starts
- admin fields and lang keys for the three settings

classes/service.go

@@ -258,6 +258,10 @@ type Feedback struct {
 	Question         models.Question
 	Result           ScoreResult
 	CorrectAnswerIDs []uint
+	// Withheld is set for an open test attempt: the solutions stay hidden
+	// and the serializer must drop Result, CorrectAnswerIDs and every
+	// solution field of Question.
+	Withheld bool
 }
 
 // Answer records or overwrites the user's answer to one question of an
@@ -334,7 +338,7 @@ func (s *Service) Answer(ctx context.Context, userID, attemptID uint, in AnswerI
 		}).Create(&row).Error; err != nil {
 			return err
 		}
-		fb = &Feedback{Question: question, Result: res, CorrectAnswerIDs: correct}
+		fb = &Feedback{Question: question, Result: res, CorrectAnswerIDs: correct, Withheld: Withheld(attempt)}
 		return nil
 	})
 	if err != nil {