Add the is-2 class to Agaric title

There's got to be an easy way...  for now, i'm overriding the template.

agaric/templates/content/page-title.html.twig

@@ -0,0 +1,19 @@
+{#
+/**
+ * @file
+ * Theme override for page titles.
+ *
+ * Available variables:
+ * - title_attributes: HTML attributes for the page title element.
+ * - title_prefix: Additional output populated by modules, intended to be
+ *   displayed in front of the main title tag that appears in the template.
+ * - title: The page title, for use in the actual content.
+ * - title_suffix: Additional output populated by modules, intended to be
+ *   displayed after the main title tag that appears in the template.
+ */
+#}
+{{ title_prefix }}
+{% if title %}
+  <h1{{ title_attributes.addClass('is-2') }}>{{ title }}</h1>
+{% endif %}
+{{ title_suffix }}