Turn letters on by default for new services

Letters is a mature enough feature now – and one that we’ve been talking
about offering for long enough – that we shouldn’t make people dig
around in the settings.

I think we’d want to wait a bit longer/indefinitely before deciding to
turn it on for existing services across the platform.

app/dao/services_dao.py

@@ -37,7 +37,8 @@ from app.models import (
     KEY_TYPE_TEST,
     NOTIFICATION_STATUS_TYPES,
     SMS_TYPE,
-    TEMPLATE_TYPES
+    TEMPLATE_TYPES,
+    LETTER_TYPE,
 )
 from app.statsd_decorators import statsd
 from app.utils import get_london_month_from_utc_column, get_london_midnight_in_utc
@@ -45,6 +46,7 @@ from app.utils import get_london_month_from_utc_column, get_london_midnight_in_u
 DEFAULT_SERVICE_PERMISSIONS = [
     SMS_TYPE,
     EMAIL_TYPE,
+    LETTER_TYPE,
     INTERNATIONAL_SMS_TYPE,
 ]