Test can create letters template

It works already, just making sure it stays working.

tests/app/dao/test_templates_dao.py

@@ -13,6 +13,7 @@ import pytest
 @pytest.mark.parametrize('template_type, subject', [
     ('sms', None),
     ('email', 'subject'),
+    ('letter', 'subject'),
 ])
 def test_create_template(sample_service, sample_user, template_type, subject):
     data = {