Add basic view to the settings page

This commit adds:
- a row to the settings page…
- …which links to a page explaining what basic view is

The new row (and link) will only appear for services who already have
the feature switched on. This is because we are not launching the
feature yet, so it shouldn’t be available to just anyone.

app/templates/views/service-settings/set-basic-view.html

@@ -0,0 +1,31 @@
+{% extends "withnav_template.html" %}
+{% from "components/textbox.html" import textbox %}
+{% from "components/page-footer.html" import page_footer %}
+
+{% block service_page_title %}
+  Basic view
+{% endblock %}
+
+{% block maincolumn_content %}
+
+  <div class="grid-row">
+    <div class="column-five-sixths">
+      <h1 class="heading-large">Basic view</h1>
+      <p>
+        Basic view lets you restrict a team member to only:
+      </p>
+      <ul class="list list-bullet">
+        <li>send messages</li>
+        <li>see sent messages</li>
+      </ul>
+      <p>
+        You’ll get to choose which team members have basic view.
+      </p>
+      {{ page_footer(
+        back_link=url_for('.service_settings', service_id=current_service.id),
+        back_link_text='Back to settings'
+      ) }}
+    </div>
+  </div>
+
+{% endblock %}