Use Template to replace/highlight placeholders

This commit brings in the `Template` util, added here:
https://github.com/alphagov/notifications-utils/pull/1

It also does a fair bit of tidying up, which I’ve unfortunately squashed into
this one massive commit. The main change is moving 404 handling into the
templates dao, so that every view isn’t littered with `try: … except(HTTPError)`.

It also adds new features, in a prototypy sort of way, which are:
- download a prefilled example CSV
- show all the columns for your template on the 'check' page

tests/app/main/views/test_sms.py

@@ -8,9 +8,8 @@ def test_choose_sms_template(app_,
                              api_user_active,
                              mock_login,
                              mock_get_user,
-                             mock_get_service_templates,
                              mock_check_verify_code,
-                             mock_get_service_template):
+                             mock_get_service_templates):
     with app_.test_request_context():
         with app_.test_client() as client:
             client.login(api_user_active)