Serialise template immediately after fetching

This commit changes the code in post notification endpoint to handle a
serialised template (ie a `dict`) rather than a database object.

This is the first step towards being able to cache the template and not
hit the database on every request.

There should be no functional changes here, it’s just refactoring.

There are some changes to the tests where the signature of functions
has changed.

Importing of the template schema has to be done at a function level,
otherwise Marshmallow gets weird.

This commit also copies the `JSONModel` class from the admin app, which
turns serialised data (a dict made from JSON) into an object on which
certain predefined properties are allowed.

This means we can still do the caching of serialised data, without
having to change too much of the code in the app, or make it ugly by
sprinkling dict lookups everywhere.

We’re not copying all of JSONModel from the admin app, just the bits we
need. We don’t need to compare or hash these objects, they’re just used
for lookups. And redefining `__getattribute__` scares Leo.

app/notifications/validators.py

@@ -8,7 +8,7 @@ from notifications_utils.recipients import (
 )
 from notifications_utils.clients.redis import rate_limit_cache_key, daily_limit_cache_key
 
-from app.dao import services_dao, templates_dao
+from app.dao import services_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, LETTER_TYPE,
@@ -21,6 +21,7 @@ from app.notifications.process_notifications import create_content_for_notificat
 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
+from app.json_models import TemplateJSONModel
 
 from gds_metrics.metrics import Histogram
 
@@ -156,11 +157,9 @@ def check_notification_content_is_not_empty(template_with_content):
 
 
 def validate_template(template_id, personalisation, service, notification_type):
+
     try:
-        template = templates_dao.dao_get_template_by_id_and_service_id(
-            template_id=template_id,
-            service_id=service.id
-        )
+        template = TemplateJSONModel.from_id_and_service_id(template_id, service.id)
     except NoResultFound:
         message = 'Template not found'
         raise BadRequestError(message=message,