Make sure confirmation/danger banners have a H1

This makes errors on all pages have a `<h1>` element, which is important
for accessibility. It means a bit of rewriting the messages, but I think
they’re better for it.

app/templates/components/banner.html

@@ -7,7 +7,7 @@
     {% endif %}
   >
     {% if subhead -%}
-        {{ subhead }} 
+      <h1 class="banner-title">{{ subhead }}</h1>
     {%- endif -%}
     {{ body }}
     {% if delete_button %}

app/templates/views/templates/template.html

@@ -29,6 +29,20 @@
     </div>
   {% endif %}
 
+  {% if template_delete_confirmation_message %}
+    <div class="bottom-gutter">
+      {% call banner_wrapper(type='dangerous', subhead=template_delete_confirmation_message[0]) %}
+        <p>
+          {{ template_delete_confirmation_message[1] }}
+        </p>
+        <form method='post'>
+          <input type="hidden" name="csrf_token" value="{{ csrf_token() }}" />
+          <input type="submit" class="button" name="delete" value="Confirm" />
+        </form>
+      {% endcall %}
+    </div>
+  {% endif %}
+
   <h1 class="heading-large">{{ template.name }}</h1>
 
   <div class="grid-row">