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,
     }
 
 

tests/app/main/views/test_service_settings.py

@@ -1850,36 +1850,36 @@ def test_set_letter_branding_saves(
     mock_update_service.assert_called_once_with(service_one['id'], dvla_organisation='500')
 
 
-def test_set_letter_class_platform_admin_only(
+def test_set_postage_platform_admin_only(
     logged_in_client,
     service_one,
 ):
-    response = logged_in_client.get(url_for('main.service_set_letter_class', service_id=SERVICE_ONE_ID))
+    response = logged_in_client.get(url_for('main.service_set_postage', service_id=SERVICE_ONE_ID))
     assert response.status_code == 403
 
 
-def test_set_letter_class_prepopulates(
+def test_set_postage_prepopulates(
     logged_in_platform_admin_client,
     service_one,
 ):
-    response = logged_in_platform_admin_client.get(url_for('main.service_set_letter_class', service_id=SERVICE_ONE_ID))
+    response = logged_in_platform_admin_client.get(url_for('main.service_set_postage', service_id=SERVICE_ONE_ID))
     assert response.status_code == 200
     page = BeautifulSoup(response.data.decode('utf-8'), 'html.parser')
     assert page.select('input[checked]')[0]['value'] == 'second'
 
 
-def test_set_letter_class_saves(
+def test_set_postage_saves(
     logged_in_platform_admin_client,
     service_one,
     mock_update_service,
 ):
     response = logged_in_platform_admin_client.post(
-        url_for('main.service_set_letter_class', service_id=SERVICE_ONE_ID),
-        data={'letter_class': 'first'}
+        url_for('main.service_set_postage', service_id=SERVICE_ONE_ID),
+        data={'postage': 'first'}
     )
     assert response.status_code == 302
     assert response.location == url_for('main.service_settings', service_id=SERVICE_ONE_ID, _external=True)
-    mock_update_service.assert_called_once_with(SERVICE_ONE_ID, letter_class='first')
+    mock_update_service.assert_called_once_with(SERVICE_ONE_ID, postage='first')
 
 
 @pytest.mark.parametrize('current_branding, expected_values, expected_labels', [