Add a hint about how to use placeholders

Since placeholders (almost) work now, it’s worth telling people what the syntax
is.

This commit also removes the ‘template type’ picker, since you can only create
SMS templates at the moment. This will be revisited when we start looking at how
you add an email template.

tests/app/main/views/test_templates.py

@@ -58,12 +58,10 @@ def test_should_redirect_when_saving_a_template(app_,
             service_id = str(uuid.uuid4())
             template_id = 456
             name = "new name"
-            type_ = "sms"
             content = "template content"
             data = {
                 'id': template_id,
                 'name': name,
-                'template_type': type_,
                 "template_content": content,
                 "service": service_id
             }
@@ -76,7 +74,7 @@ def test_should_redirect_when_saving_a_template(app_,
             assert response.location == url_for(
                 '.manage_service_templates', service_id=service_id, _external=True)
             mock_update_service_template.assert_called_with(
-                template_id, name, type_, content, service_id)
+                template_id, name, 'sms', content, service_id)
 
 
 def test_should_show_delete_template_page(app_,