Refactor

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

playlist/models/artist.py

@@ -0,0 +1,9 @@
+from django.db import models
+
+class Artist(models.Model):
+    def __str__(self):
+        return self.name
+    id = models.CharField(primary_key=True, max_length=128)
+    name = models.CharField(max_length=255)
+    popularity = models.SmallIntegerField(null=True, blank=True)
+    genres = models.JSONField(null=True)