Rename JSONModel to SerialisedModel 1/2

This class doesn’t actually wrap JSON, it wraps serialised data.

So this name feels better.

This commit only renames the file for an easier diff.

tests/app/notifications/test_validators.py

@@ -23,7 +23,7 @@ from app.notifications.validators import (
     validate_and_format_recipient,
     validate_template,
 )
-from app.json_models import TemplateJSONModel
+from app.serialised_models import TemplateJSONModel
 from app.utils import get_template_instance
 
 from app.v2.errors import (