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/templates/views/service-settings.html

@@ -238,11 +238,11 @@
         {% if current_user.platform_admin %}
           {% call row() %}
             {{ text_field('Postage') }}
-              {% set letter_class = {'first': 'First class only', 'second': 'Second class only'} %}
-              {{ text_field(letter_class[current_service.letter_class]) }}
+              {% set postage = {'first': 'First class only', 'second': 'Second class only'} %}
+              {{ text_field(postage[current_service.postage]) }}
             {{ edit_field(
                 'Change',
-                url_for('.service_set_letter_class',
+                url_for('.service_set_postage',
                 service_id=current_service.id),
                 permissions=['manage_service']
             )

app/templates/views/service-settings/set-postage.html

@@ -26,7 +26,7 @@
         Royal Mail delivers from Monday to Saturday, excluding bank holidays.
       </p>
       <form method="post">
-        {{ radios(form.letter_class) }}
+        {{ radios(form.postage) }}
         {{ page_footer(
           'Save',
           back_link=url_for('.service_settings', service_id=current_service.id),