Let users switch channels on and off by themselves

We let people do this for letters already. We should let them do it for
emails and texts too, rather than have to email us.

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

@@ -0,0 +1,33 @@
+{% extends "withnav_template.html" %}
+{% from "components/radios.html" import radios %}
+{% from "components/page-footer.html" import page_footer %}
+{% from "components/form.html" import form_wrapper %}
+
+{% block service_page_title %}
+  Send letters
+{% endblock %}
+
+{% block maincolumn_content %}
+
+  <div class="grid-row">
+    <div class="column-five-sixths">
+      <h1 class="heading-large">Send letters</h1>
+      <p>
+        It costs between 30p and 76p to send a letter using Notify.
+      </p>
+      <p>
+        See <a href="{{ url_for(".pricing", _anchor="letters") }}">pricing</a> for the list
+        of rates.
+      </p>
+      {% call form_wrapper() %}
+        {{ radios(form.enabled) }}
+        {{ page_footer(
+          'Save',
+          back_link=url_for('.service_settings', service_id=current_service.id),
+          back_link_text='Back to settings'
+        ) }}
+      {% endcall %}
+    </div>
+  </div>
+
+{% endblock %}