Improve quiz layout: add fixed topbar, include navigation, and clean up unused elements

- Add `.quiz-topbar` with fixed positioning and spacing in `quiz.scss`
- Update `base.html.twig` to include `quiz/nav.html.twig` in a new `nav` block
- Remove unused "Manage Quiz" button from `select_season.html.twig`

templates/quiz/select_season.html.twig

@@ -2,6 +2,5 @@
 {% block body %}
     <div class="quiz-form-narrow">
         {{ form(form) }}
-        <a href="{{ path('tvdt_backoffice_index') }}" class="btn btn-outline-secondary">{{ 'Manage Quiz'|trans }}</a>
     </div>
 {% endblock body %}