Add validators for service_letter_contact_id and reply_to

Validators check that service_letter_contact_id belongs to the
same service as the notification/template.

Generic reply_to validator calls the correct function for the given
type (for either notification or template). It can be used by the
template API endpoints to verify that given reply_to ID has the same
service_id as the template itself.

The original approach was to create a DB foreign key constraint,
but this caused issues with the `version_class` decorator saving
related Service objects without creating a history record.

app/notifications/validators.py

@@ -10,7 +10,7 @@ from notifications_utils.clients.redis import rate_limit_cache_key, daily_limit_
 from app.dao import services_dao, templates_dao
 from app.dao.service_sms_sender_dao import dao_get_service_sms_senders_by_id
 from app.models import (
-    INTERNATIONAL_SMS_TYPE, SMS_TYPE, EMAIL_TYPE,
+    INTERNATIONAL_SMS_TYPE, SMS_TYPE, EMAIL_TYPE, LETTER_TYPE,
     KEY_TYPE_TEST, KEY_TYPE_TEAM, SCHEDULE_NOTIFICATIONS
 )
 from app.service.utils import service_allowed_to_send_to
@@ -20,6 +20,7 @@ from app import redis_store
 from app.notifications.process_notifications import create_content_for_notification
 from app.utils import get_public_notify_type_text
 from app.dao.service_email_reply_to_dao import dao_get_reply_to_by_id
+from app.dao.service_letter_contact_dao import dao_get_letter_contact_by_id
 
 
 def check_service_over_api_rate_limit(service, api_key):
@@ -141,6 +142,15 @@ def validate_template(template_id, personalisation, service, notification_type):
     return template, template_with_content
 
 
+def check_reply_to(service_id, reply_to_id, type_):
+    if type_ == EMAIL_TYPE:
+        return check_service_email_reply_to_id(service_id, reply_to_id, type_)
+    elif type_ == SMS_TYPE:
+        return check_service_sms_sender_id(service_id, reply_to_id, type_)
+    elif type_ == LETTER_TYPE:
+        return check_service_letter_contact_id(service_id, reply_to_id, type_)
+
+
 def check_service_email_reply_to_id(service_id, reply_to_id, notification_type):
     if reply_to_id:
         try:
@@ -159,3 +169,13 @@ def check_service_sms_sender_id(service_id, sms_sender_id, notification_type):
             message = 'sms_sender_id {} does not exist in database for service id {}'\
                 .format(sms_sender_id, service_id)
             raise BadRequestError(message=message)
+
+
+def check_service_letter_contact_id(service_id, letter_contact_id, notification_type):
+    if letter_contact_id:
+        try:
+            return dao_get_letter_contact_by_id(service_id, letter_contact_id).contact_block
+        except NoResultFound:
+            message = 'letter_contact_id {} does not exist in database for service id {}'\
+                .format(letter_contact_id, service_id)
+            raise BadRequestError(message=message)