Prefill placeholders for test message

If you want to send yourself a test message from a template that has
placeholders you can’t, at the moment.

Rather than forcing you to upload a CSV, we should prefil the data, and then
you only need to upload a CSV if you want to customise it.

app/main/views/sms.py

@@ -104,10 +104,12 @@ def get_example_csv(service_id, template_id):
 @main.route("/services/<service_id>/sms/send/<template_id>/to-self", methods=['GET'])
 @login_required
 def send_sms_to_self(service_id, template_id):
+    template = templates_dao.get_service_template_or_404(service_id, template_id)['data']
+    placeholders = list(Template(template).placeholders)
     output = io.StringIO()
     writer = csv.writer(output)
-    writer.writerow(['phone'])
-    writer.writerow([current_user.mobile_number])
+    writer.writerow(['phone'] + placeholders)
+    writer.writerow([current_user.mobile_number] + ["test {}".format(header) for header in placeholders])
     filedata = {
         'file_name': 'Test run',
         'data': output.getvalue().splitlines()