refs KTLO-19
When we need to migrate subscriptions from a platform with platform
fees, we need to recreate the subscriptions. That can cause the same
subscription to be attached multiple times to the same member in Ghost.
This is a problem because all MRR, subscriptions and cancellations stats
are no longer correct. Ghost will add a MRR event for the duplicated
subscription from the start time, so there is a sudden peak in MRR and a
dip after the migration because all those duplicate subscriptions are
suddenly cancelled 'today'.
The migrator tool adds a ghost_migrated_to metadata field to the old
subscription. Ghost can use this to detect the old subscription and
delete the subscription and corresponding events.
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