Use data-quiz-shown attributes for quiz mode visibility

Mark elements that should remain visible in quiz mode at build time
via data-quiz-shown attributes (toolbar, breadcrumb, h1, questions,
solution links, next-section/-chapter links). The runtime tail
detection becomes obsolete and the CSS collapses to a single show/hide
rule.

Switch the body markers to data-quiz-mode / data-quiz-has-questions
attributes, swap the toggle between btn-secondary and btn-primary
instead of hardcoded colors, and use Bootstrap's alert alert-info for
the no-questions notice.

templates/html/question.html

@@ -1,4 +1,4 @@
-<div class="question layout-{{ layout }}" id="{{ number }}">
+<div class="question layout-{{ layout }}" id="{{ number }}" data-quiz-shown>
   <div class="question-text">
     {{ number }}: {{ question }}
   </div>
@@ -23,7 +23,7 @@
     {% endfor %}
   </div>
 </div>
-<div class="solution">
+<div class="solution" data-quiz-shown>
   {% if has_solution %}
     <a class="btn btn-warning btn-sm d-inline-flex align-items-center" href="{{ number }}.html" aria-label="Lösungshinweis zu Frage {{ number }}" title="Lösungshinweis">
       <svg class="solution-icon" xmlns="http://www.w3.org/2000/svg" viewBox="0 0 640 640" fill="currentColor" aria-hidden="true" focusable="false">