templates: allow content before and after a2 block (#54471)

templates/authentic2/base-page.html

@@ -1,6 +1,7 @@
 {% extends "authentic2/base.html" %}
 
 {% block placeholder-content %}
+{% block a2-block-pre %}{% endblock %}
 <div class="a2-block">
   {% block form-title %}
   {% if view.title %}<h2>{{ view.title }}</h2>{% endif %}
@@ -8,4 +9,5 @@
   {% block content %}
   {% endblock %}
 </div>
+{% block a2-block-post %}{% endblock %}
 {% endblock %}