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.

app/notify_client/models.py

@@ -275,12 +275,12 @@ class Service(dict):
         'email_from',
         'id',
         'inbound_api',
-        'letter_class',
         'letter_contact_block',
         'message_limit',
         'name',
         'organisation_type',
         'permissions',
+        'postage',
         'prefix_sms',
         'research_mode',
         'service_callback_api',