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.

app/serialised_models.py

@@ -0,0 +1,30 @@
+from abc import ABC, abstractmethod
+
+
+class JSONModel(ABC):
+
+    @property
+    @abstractmethod
+    def ALLOWED_PROPERTIES(self):
+        pass
+
+    def __init__(self, _dict):
+        for property in self.ALLOWED_PROPERTIES:
+            setattr(self, property, _dict[property])
+
+    def __dir__(self):
+        return super().__dir__() + list(sorted(self.ALLOWED_PROPERTIES))
+
+
+class TemplateJSONModel(JSONModel):
+    ALLOWED_PROPERTIES = {
+        'archived',
+        'content',
+        'id',
+        'postage',
+        'process_type',
+        'reply_to_text',
+        'subject',
+        'template_type',
+        'version',
+    }