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,