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/main/views/templates.py

@@ -428,22 +428,24 @@ def delete_service_template(service_id, template_id):
         last_used_notification = template_statistics_client.get_template_statistics_for_template(
             service_id, template['id']
         )
-        message = '{} was last used {} ago'.format(
-            last_used_notification['template']['name'],
+        message = 'It was last used {} ago.'.format(
             get_human_readable_delta(
                 parse(last_used_notification['created_at']).replace(tzinfo=None),
-                datetime.utcnow())
+                datetime.utcnow()
+            )
         )
     except HTTPError as e:
         if e.status_code == 404:
-            message = '{} has never been used'.format(template['name'])
+            message = 'It’s never been used.'.format(template['name'])
         else:
             raise e
 
-    flash('{}. Are you sure you want to delete it?'.format(message), 'delete')
-
     return render_template(
         'views/templates/template.html',
+        template_delete_confirmation_message=(
+            'Are you sure you want to delete {}?'.format(template['name']),
+            message,
+        ),
         template=get_template(
             template,
             current_service,

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">