fix: Convert block_structure.data to relative paths (TNL-8335)

In order to upgrade to Django > 2.2.20, we can't continue to use
absolute paths in the block_structure's data FileField. This used to
work for S3, but it will not work going forward due to a security fix
in Django 2.2.21.

This data migration will remove the starting '/' from any paths in the
block_structure table. The resulting locations in S3 should be
unaffected.

openedx/core/djangoapps/content/block_structure/migrations/0005_trim_leading_slashes_in_data_path.py

@@ -0,0 +1,46 @@
+"""
+Data migration to convert absolute paths in block_structure.data to be relative.
+
+This has only been tested with MySQL, though it should also work for Postgres as
+well. This is necessary to manually correct absolute paths in the "data" field
+of the block_structure table. For S3 storage, having a path that starts with
+"/courses/" puts things in the same place as a path starting with "courses/",
+but absolute paths are not permitted for FileFields.
+
+These values would have always been broken in devstack (because it's not in
+MEDIA_ROOT), but it used to work for the S3 storages option because the security
+checking happened at the storage layer, and the path is equivalent in S3 because
+we just append either value to the bucket's root.
+
+However, in Django > 2.2.20, this checking against absolute paths has been added
+to the FileField itself, and an upgrade attempt started causing write failures
+to Block Structures.
+
+There are separate PRs to fix the config values so that new writes start with a
+"courses/" prefix. This migration to is fix old entries by removing any leading
+"/" characters.
+
+THIS MIGRATION MUST BE RUN BEFORE UPGRADING TO DJANGO > 2.2.20 IF YOU ARE
+USING A STORAGE_CLASS IN BLOCK_STRUCTURES_SETTINGS. If you do not specify this
+setting and only run Block Structures out of memcached, this should not affect
+you.
+"""
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('block_structure', '0004_blockstructuremodel_usagekeywithrun'),
+    ]
+
+    operations = [
+        migrations.RunSQL(
+            """
+            UPDATE block_structure
+                SET data = right(data, length(data) - 1)
+                WHERE data like '/%';
+            """
+        )
+    ]