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/process_letter_notifications.py

@@ -6,7 +6,14 @@ from app.notifications.process_notifications import persist_notification
 
 
 def create_letter_notification(
-    letter_data, template, api_key, status, reply_to_text=None, billable_units=None, updated_at=None
+    letter_data,
+    template,
+    service,
+    api_key,
+    status,
+    reply_to_text=None,
+    billable_units=None,
+    updated_at=None,
 ):
     notification = persist_notification(
         template_id=template.id,
@@ -14,7 +21,7 @@ def create_letter_notification(
         template_postage=template.postage,
         # we only accept addresses_with_underscores from the API (from CSV we also accept dashes, spaces etc)
         recipient=PostalAddress.from_personalisation(letter_data['personalisation']).normalised,
-        service=template.service,
+        service=service,
         personalisation=letter_data['personalisation'],
         notification_type=LETTER_TYPE,
         api_key_id=api_key.id,