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

tests/app/main/views/organisations/test_organisation_invites.py

@@ -66,7 +66,7 @@ def test_view_organisation_shows_the_correct_organisation(
         org_id=ORGANISATION_ID,
     )
 
-    assert normalize_spaces(page.select_one('.heading-large').text) == 'Services'
+    assert normalize_spaces(page.select_one('h1').text) == 'Services'
 
 
 def test_create_new_organisation(