fixes https://github.com/TryGhost/Team/issues/2607
When a free trial converts to a paid subscription, and increases the MRR, it just creates a 'updated' paid subscription event.
To fix this, we need to count updated events that didn't change plan but do have a positive MRR. As an extension we could also check if the MRR change matches the expected MRR for the corresponsing Stripe plan, but that requires a more complex condition check (because for yearly subscriptions we need to convert to monthly), I don't think that is required here.
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