Backfill subscription realm audit log SUBSCRIPTION_CREATED events for
users which are currently subscribed but don't have any subscription
events, presumably due to some historical bug. This is important
because those rows are necessary when reactivating a user who is
currently soft-deactivated.
For each stream, we find the subscribed users who have no
subscription-related realm audit log entries, and create a
`backfill=True` subscription audit log entry which is the latest it
could have been, based on UserMessage rows. We then optionally insert
a `DEACTIVATION` if the current subscription is not active.
Alex Vandiver