Normalise heading sizes

Since we added template folders the templates page has had a ‘medium’
sized heading, where other pages have stuck with a ‘large’ size.

This commit rationalises the decision around which pages have which
heading size:
- ‘navigation’ pages (eg templates, team members, email reply to
  addresses) have medium sized headings
- transactional pages (ie ones which have a green button) keep the
  larger heading size

app/templates/views/usage.html

@@ -11,7 +11,7 @@
 
 {% block maincolumn_content %}
 
-    <h1 class='heading-large'>Usage</h1>
+    <h1 class='heading-medium'>Usage</h1>
 
     <div class="bottom-gutter">
       {{ pill(years, selected_year, big_number_args={'smallest': True}) }}