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/organisations/organisation/users/index.html

@@ -11,7 +11,7 @@
 
 {% block maincolumn_content %}
 
-  <h1 class="heading-large">
+  <h1 class="heading-medium">
     Team members
   </h1>