Don’t store the underlying dict

This will give us smaller objects to cache, and forces us to be explicit
about which properties we’re using.

app/json_models.py

@@ -11,7 +11,6 @@ class JSONModel(ABC):
         pass
 
     def __init__(self, _dict):
-        self._dict = _dict
         for property in self.ALLOWED_PROPERTIES:
             setattr(self, property, _dict[property])
 
@@ -22,10 +21,12 @@ class JSONModel(ABC):
 class TemplateJSONModel(JSONModel):
     ALLOWED_PROPERTIES = {
         'archived',
+        'content',
         'id',
         'postage',
         'process_type',
         'reply_to_text',
+        'subject',
         'template_type',
         'version',
     }

app/notifications/process_notifications.py

@@ -50,12 +50,29 @@ REDIS_GET_AND_INCR_DAILY_LIMIT_DURATION_SECONDS = Histogram(
 
 def create_content_for_notification(template, personalisation):
     if template.template_type == EMAIL_TYPE:
-        template_object = PlainTextEmailTemplate(template._dict, personalisation)
+        template_object = PlainTextEmailTemplate(
+            {
+                'content': template.content,
+                'subject': template.subject,
+                'template_type': template.template_type,
+            },
+            personalisation,
+        )
     if template.template_type == SMS_TYPE:
-        template_object = SMSMessageTemplate(template._dict, personalisation)
+        template_object = SMSMessageTemplate(
+            {
+                'content': template.content,
+                'template_type': template.template_type,
+            },
+            personalisation,
+        )
     if template.template_type == LETTER_TYPE:
         template_object = LetterPrintTemplate(
-            template._dict,
+            {
+                'content': template.content,
+                'subject': template.subject,
+                'template_type': template.template_type,
+            },
             personalisation,
             contact_block=template.reply_to_text,
         )