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/api/index.html

@@ -10,7 +10,7 @@
 
 {% block maincolumn_content %}
 
-  <h1 class="heading-large bottom-gutter">
+  <h1 class="heading-medium bottom-gutter-3-2">
     API integration
   </h1>
 
@@ -28,7 +28,7 @@
 
   <div class="grid-row">
     <div class="column-half">
-      <h2 class="heading-medium">
+      <h2 class="heading-small">
         Message log
       </h2>
     </div>