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

lang/en/lang.yaml

@@ -34,6 +34,16 @@ quizzes:
     round_size: Round size
     round_size_comment: Clients may present the questions in rounds of this many.
     shuffle_questions: Shuffle questions
+    mode: Mode
+    mode_learning: Learning
+    mode_test: Test
+    mode_comment: A test hides the correct answers and explanations until the attempt is finished.
+    navigation: Test navigation
+    navigation_free: Free (go back and change answers)
+    navigation_one_way: One way (an answer is final)
+    navigation_comment: Applies to tests only.
+    max_attempts: Attempt limit
+    max_attempts_comment: Tests only. Leave empty for no limit.
 questions:
     title: Questions
     question: Question

lang/pl/lang.yaml

@@ -34,6 +34,16 @@ quizzes:
     round_size: Wielkość rundy
     round_size_comment: Klient może pokazywać pytania w rundach po tyle pytań.
     shuffle_questions: Losowa kolejność pytań
+    mode: Tryb
+    mode_learning: Nauka
+    mode_test: Test
+    mode_comment: Test ukrywa poprawne odpowiedzi i wyjaśnienia do zakończenia podejścia.
+    navigation: Nawigacja w teście
+    navigation_free: Swobodna (można wracać i zmieniać odpowiedzi)
+    navigation_one_way: Jednokierunkowa (odpowiedź jest ostateczna)
+    navigation_comment: Dotyczy tylko testów.
+    max_attempts: Limit podejść
+    max_attempts_comment: Tylko dla testów. Zostaw puste, aby nie ograniczać.
 questions:
     title: Pytania
     question: Pytanie