Make send a test teach you how placeholders work

This commit does two main things:

- adds textboxes to the send yourself a test page, so you can replace
  ((name)) with ‘Chris’, or whatever your name is
- rejigs the send page a bit to make it clearer what the CSV upload is
  for and how to use it

The idea being that, since most users go into ‘send yourself a test’
first, it teaches them about how placeholders work by making them do the
replacing themselves.

tests/conftest.py

@@ -202,6 +202,18 @@ def mock_get_service_template(mocker):
         'app.service_api_client.get_service_template', side_effect=_get)
 
 
+@pytest.fixture(scope='function')
+def mock_get_service_template_with_placeholders(mocker):
+    def _get(service_id, template_id):
+        template = template_json(
+            service_id, template_id, "Two week reminder", "sms", "((name)), your vehicle tax is about to expire"
+        )
+        return {'data': template}
+
+    return mocker.patch(
+        'app.service_api_client.get_service_template', side_effect=_get)
+
+
 @pytest.fixture(scope='function')
 def mock_get_service_email_template(mocker):
     def _create(service_id, template_id):