Add page to change SMS allowance

There may be exceptions to the rule, so we should have a way of
overriding this manually.

app/templates/views/service-settings.html

@@ -198,7 +198,7 @@
         {% call row() %}
           {{ text_field('Free text message allowance')}}
           {{ text_field('{:,}'.format(current_service.free_sms_fragment_limit)) }}
-          {{ text_field() }}
+          {{ edit_field('Change', url_for('.set_free_sms_allowance', service_id=current_service.id)) }}
         {% endcall %}
         {% call row() %}
           {{ text_field('Email branding' )}}

app/templates/views/service-settings/set-free-sms-allowance.html

@@ -0,0 +1,21 @@
+{% extends "withnav_template.html" %}
+{% from "components/textbox.html" import textbox %}
+{% from "components/page-footer.html" import page_footer %}
+
+{% block service_page_title %}
+  Emails
+{% endblock %}
+
+{% block maincolumn_content %}
+
+  <form method="post">
+    <h1 class="heading-large">Free text message allowance</h1>
+    {{ textbox(form.free_sms_allowance) }}
+    {{ page_footer(
+      'Save',
+      back_link=url_for('.service_settings', service_id=current_service.id),
+      back_link_text='Back to settings'
+    ) }}
+  </form>
+
+{% endblock %}