feat: add submission assignee model

pretalx_musicrate/models.py

@@ -105,3 +105,10 @@ class Rating(models.Model):
                 fields=("submission", "juror"), name="unique_rating"
             ),
         ]
+
+
+class Assignee(models.Model):
+    submission = models.OneToOneField("submission.Submission", on_delete=models.CASCADE)
+    user = models.ForeignKey(
+        "person.User", on_delete=models.CASCADE, related_name="assigned_submissions"
+    )