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.