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 %}