Merge branch 'feature/ISTWCMS-4847-ebremner-404-pages' into '3.0.x'

ISTWCMS-4847: allowing classes in the main content region

See merge request !59

source/_patterns/03-layouts/content/content.twig

-<main id="main" class="uw-main" role="main" tabindex="-1">
+{% set main_classes = 'uw-main' %}
 
-    {% block content %}
+{% for class in classes %}
+  {% set main_classes = main_classes ~ ' ' ~ class %}
+{% endfor %}
+
+<main id="main" class="{{ main_classes }}" role="main" tabindex="-1">
+  {% block content %}
+    Main content
   {% endblock %}
 </main>