We should have done this a long time ago, but better late than never.
Basically, this migration would crash in the event that there were any
attachments with particularly long names. The fix is the next
migration, 0042; we just inline it here to avoid that crash.
This migration ensures that all historically uploaded files from
before we started tracking files in the Zulip database via the
Attachment model have Attachment objects.
This has been tested by tabbott against a production server to ensure
that it results in all old uploaded files having corresponding
attachment objects.
Merging this change is a key prerequisite for making our adding
attachment access controls in an enforcing fashion.
