Add a page to pick a template to reply with

If your service is receiving inbound messages, you might want to reply
to them. Without an API integration, the only way to do this is by doing
the send one off flow. But currently there is no direct link from the
page which shows the inbound messages.

So this commit:
- adds a link to a new page…
- …which shows the available (ie only text message) templates with which
  you can reply

app/templates/views/conversations/conversation.html

@@ -22,6 +22,12 @@
       'messages',
     ) }}
 
+    {% if current_user.has_permissions(['send_texts'], admin_override=True) %}
+      <p>
+        <a href="{{ url_for('.conversation_reply', service_id=current_service.id, notification_id=notification_id) }}">Send a text message to this phone number</a>
+      </p>
+    {% endif %}
+
   </div>
 
 {% endblock %}