Implement quizToXlsx() export and add export button

- QuizSpreadsheetService: implement quizToXlsx() as the inverse of
  fillQuizFromArray() — writes quiz questions and answers to XLSX using
  the same column layout as the import template
- BackofficeController: add exportQuiz() action at GET /backoffice/quiz/{quiz}/export
- tab_overview.html.twig: add Export to XLSX button in Quick actions

templates/backoffice/quiz/tab_overview.html.twig

@@ -49,6 +49,11 @@
         </button>
     </div>
 
+    <a class="btn btn-outline-secondary mb-3"
+       href="{{ path('tvdt_backoffice_quiz_export', {quiz: quiz.id}) }}">
+        {{ 'Export to XLSX'|trans }}
+    </a>
+
     <h4 class="mb-3">{{ 'Questions'|trans }}</h4>
     <div class="accordion">
         {%~ for question in quiz.questions ~%}