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):

app/main/views/service_settings.py

@@ -42,6 +42,7 @@ from app.main.forms import (
     ServiceDataRetentionForm,
     ServiceEditInboundNumberForm,
     ServiceInboundNumberForm,
+    ServiceLetterClassForm,
     ServiceLetterContactBlockForm,
     ServicePreviewBranding,
     ServiceReplyToEmailForm,
@@ -649,6 +650,19 @@ def service_set_letters(service_id):
     )
 
 
+@main.route("/services/<service_id>/service-settings/set-letter-class", methods=['GET', 'POST'])
+@login_required
+@user_is_platform_admin
+def service_set_letter_class(service_id):
+    form = ServiceLetterClassForm(letter_class=current_service.letter_class)
+
+    if form.validate_on_submit():
+        service_api_client.update_service(service_id, letter_class=form.letter_class.data)
+        return redirect(url_for(".service_settings", service_id=service_id))
+
+    return render_template('views/service-settings/set-letter-class.html', form=form)
+
+
 @main.route("/services/<service_id>/service-settings/set-auth-type", methods=['GET'])
 @login_required
 @user_has_permissions('manage_service')