Add settings page for inbound SMS

Users might be interested in inbound SMS. And when it’s fully
available, they’ll probably be able to control whether it’s on/off for
their service.

Until they point, the only way of getting it is to ask us. So let’s make
an in-the-meantime page that directs them to ask us, from the place
where they’d be able to do it themselves.

app/templates/views/service-settings.html

@@ -51,6 +51,12 @@
           {{ edit_field('Change', url_for('.service_set_international_sms', service_id=current_service.id)) }}
         {% endcall %}
 
+        {% call row() %}
+          {{ text_field('Receive text messages') }}
+          {{ text_field('On' if 'inbound_sms' in current_service.permissions else 'Off') }}
+          {{ edit_field('Change', url_for('.service_set_inbound_sms', service_id=current_service.id)) }}
+        {% endcall %}
+
         {% call row() %}
           {{ text_field('Letters') }}
           {{ text_field('On' if current_service.can_send_letters else 'Off') }}