Refactor

Moves from_json to utils and splits the models to files in a module

playlist/models/utils/json.py

@@ -0,0 +1,8 @@
+def from_json(cls, json):
+    try:
+        return cls.objects.get(pk=json["id"])
+    except cls.DoesNotExist:
+        keys = [f.name for f in cls._meta.get_fields()]
+        subset = {key:json[key] for key in set(keys) & set(json.keys())}
+        return cls.objects.create(**subset)
+