Add letter class row to settings page, visible to platform admin

Added a new row to the settings table, 'Post class', which shows the
default letter class of a service and is only visible to Platform Admin.

Also added a new page to enable Platform Admin users to change the
default letter class for a service - this only has two options at the
moment, 1st class only and 2nd class only.

tests/__init__.py

@@ -139,6 +139,7 @@ def service_json(
     email_branding=None,
     branding='govuk',
     created_at=None,
+    letter_class='second',
     letter_contact_block=None,
     inbound_api=None,
     service_callback_api=None,
@@ -168,6 +169,7 @@ def service_json(
         'email_branding': email_branding,
         'branding': branding,
         'created_at': created_at or str(datetime.utcnow()),
+        'letter_class': letter_class,
         'letter_contact_block': letter_contact_block,
         'dvla_organisation': '001',
         'permissions': permissions,