Allow override of top-level container class.

sourcing/templates/base.html

@@ -18,7 +18,7 @@
 {% include "navbar.html" %}
 {% block dialogs %}
 {% endblock %}
-<div class="container-fluid mt-3">
+<div class="{% block container_class %}container-fluid{% endblock %} mt-3">
 {% include "flash_msg.html" %}
 
 {% block content %}
@@ -29,7 +29,6 @@
 </div>
 
 <script src="{{ url_for('static', filename='jquery/jquery.js') }}"></script>
-<script src="https://npmcdn.com/tether@1.2.4/dist/js/tether.min.js"></script>
 <script src="{{ url_for('static', filename='bootstrap4/js/bootstrap.js') }}"></script>
 
 {% block scripts %}

sourcing/templates/form/simple.html

@@ -1,5 +1,7 @@
 {% extends "base.html" %}
 
+{% block container_class %}container{% endblock %}
+
 {% block content %}
   <h1>{{ self.title() }}</h1>
   {% include "form/main.html" %}