fix: Make container_type backfill reversible

This is trivial and will be helpful for devs who need to
move back and forth across this migrations step for
whatever reason (switching branches, resolving DB issues).

Author: kdmccormick

src/openedx_content/migrations/0005_containertypes.py

@@ -29,6 +29,14 @@ def backfill_container_types(apps, schema_editor):
         raise ValueError(f"container {unknown_containers[0]} is of unknown container type. Cannot apply migration.")
 
 
+def clear_container_types(apps, schema_editor):
+    """
+    Drop the contents of the container_type field for the reverse migration.
+    """
+    Container = apps.get_model("openedx_content", "Container")
+    Container.objects.update(container_type=None)
+
+
 class Migration(migrations.Migration):
     dependencies = [
         ("openedx_content", "0004_componenttype_constraint"),
@@ -68,7 +76,10 @@ class Migration(migrations.Migration):
             ),
         ),
         # 3. Populate the container_type column, which is currently NULL for all existing containers
-        migrations.RunPython(backfill_container_types),
+        migrations.RunPython(
+            code=backfill_container_types,
+            reverse_code=clear_container_types,
+        ),
         # 4. disallow NULL values from now on
         migrations.AlterField(
             model_name="container",