Add frozen subtasks

judge/models/contest.py

@@ -774,6 +774,13 @@ class ContestProblem(models.Model):
             MinValueValidator(0, _("Why include a problem you " "can't submit to?"))
         ],
     )
+    frozen_subtasks = models.CharField(
+        help_text=_("Only for format new IOI. Separated by commas, e.g: 2, 3"),
+        verbose_name=_("frozen subtasks"),
+        null=True,
+        blank=True,
+        max_length=20,
+    )
 
     @property
     def clarifications(self):