Change naming in code to use 'postage'

Renamed everything that was previously called 'letter class' or 'postage
class' to 'postage', which is the new name we have decided to use.

tests/__init__.py

@@ -139,7 +139,6 @@ 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,
@@ -147,6 +146,7 @@ def service_json(
     organisation_type='central',
     prefix_sms=True,
     contact_link=None,
+    postage='second',
 ):
     if users is None:
         users = []
@@ -169,13 +169,13 @@ 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,
         'inbound_api': inbound_api,
         'service_callback_api': service_callback_api,
         'prefix_sms': prefix_sms,
+        'postage': postage,
     }