Override bulma message template to add styling classes

... also we're presuming we're going to customize this more later.

agaric/templates/system/status-messages.html.twig

@@ -0,0 +1,54 @@
+{#
+/**
+ * @file
+ * Default theme implementation for status messages.
+ *
+ * Displays status, error, and warning messages, grouped by type.
+ *
+ * An invisible heading identifies the messages for assistive technology.
+ * Sighted users see a colored box. See http://www.w3.org/TR/WCAG-TECHS/H69.html
+ * for info.
+ *
+ * Add an ARIA label to the contentinfo area so that assistive technology
+ * user agents will better describe this landmark.
+ *
+ * Available variables:
+ * - message_list: List of messages to be displayed, grouped by type.
+ * - status_headings: List of all status types.
+ * - display: (optional) May have a value of 'status' or 'error' when only
+ *   displaying messages of that specific type.
+ * - attributes: HTML attributes for the element, including:
+ *   - class: HTML classes.
+ *
+ * @ingroup themeable
+ */
+#}
+
+{% for type, messages in message_list %}
+  {%- set classes = [
+    'message',
+    type == 'error' ? 'is-danger',
+    type == 'status' ? 'is-success',
+    type == 'warning' ? 'is-warning',
+    'is-mono',
+    'is-size-4-tablet',
+    'is-size-5',
+  ] -%}
+  <div {{ attributes.addClass(classes) }} role="contentinfo" aria-label="{{ status_headings[type] }}"{{ attributes|without('role', 'aria-label') }}>
+    <div class="{{ type }}"></div>
+    {% if status_headings[type] %}
+      <h2 class="visually-hidden">{{ status_headings[type] }}</h2>
+    {% endif %}
+    <div class="message-body">
+    {% if messages|length > 1 %}
+      <ul>
+        {% for message in messages %}
+          <li>{{ message }}</li>
+        {% endfor %}
+      </ul>
+    {% else %}
+      {{ messages|first }}
+    {% endif %}
+    </div>
+  </div>
+{% endfor %}