Resolve TMSS-780

Closes TMSS-780

SAS/TMSS/backend/src/tmss/tmssapp/migrations/0004_subtask_obsolete_since.py

+# Generated by Django 3.0.9 on 2021-10-12 07:07
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('tmssapp', '0003_taskblueprint_shortdescription'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='subtask',
+            name='obsolete_since',
+            field=models.DateTimeField(help_text='When this subtask was marked obsolete, or NULL if not obsolete (NULLable).', null=True),
+        ),
+    ]