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.

app/main/forms.py

@@ -684,6 +684,17 @@ class ServiceSwitchLettersForm(StripWhitespaceForm):
     )
 
 
+class ServiceLetterClassForm(StripWhitespaceForm):
+
+    letter_class = RadioField(
+        'Postage classes',
+        choices=[
+            ('first', '1st class only'),
+            ('second', '2nd class only'),
+        ],
+    )
+
+
 class BrandingStyle(RadioField):
 
     def post_validate(self, form, validation_stopped):